Title: Effect of the Interval Between Pregnancies on Labor Dystocia

Editorial Manager(tm) for American Journal of Obstetrics and Gynecology Manuscript Draft Manuscript Number: W05-0446R1 Title: Effect of the Interval Between Pregnancies on Labor Dystocia Article Type: Regular Keywords: interpregnancy interval; birth intervals; dystocia; difficult childbirth; difficult labor. Corresponding Author: Dr. Bao-Ping Zhu, MD, MS Corresponding Author's Institution: MO Dept Hlth & Senior Services First Author: Bao-Ping Zhu, MD, MS Order of Authors: Bao-Ping Zhu, MD, MS; Violanda Grigorescu, MD, MSPH; Thu Le, PhD; Mei Lin, MD, MSc; Glenn Copeland, MBA; Maurice Barone; George Turabelidze, MD, PhD

October 18, 2005 Editor, American Journal of Obstetrics and Gynecology 5228 Bressler Dr. Hilliard, OH 43026 Re: Manuscript W05-0446 Dear Editor: Thank you for sending us the reviewers’ comments for our manuscript, “Effect of the Interval Between Pregnancies on Labor Dystocia.” We appreciate both reviewers for their kind words and encouragements. We have revised our manuscript based on their insightful and constructive comments, and explained where changes have been made in my response in the following two pages. We have also made some other editorial changes to make the manuscript read better, and corrected a few minor errors in the original version of the manuscript. All authors of the original manuscript have read and approved the revised version of the paper. Please continue to address all correspondence to Dr. Bao-Ping Zhu, State Epidemiologist, Missouri Department of Health and Senior Services, 912 Wildwood Drive, P.O. Box 570, Jefferson City, MO 65102, Telephone: (573) 751-6128; Fax: (573) 522-6003; email: [email protected]. Thank you for your kind review. We look forward to hearing the decision by the Journal regarding our paper. Sincerely yours, Bao-Ping Zhu, MD, MS Enclosures.

Cover Letter for revised paper

Reviewers’ Comments and My Response (Highlighted): Reviewer #1: I would like to congratulate the authors on preparing an excellent piece of work. The literature is rich with publications discussing the benefits of longer interpregnancy intervals. This article points out one of the few potential risks of increased intervals. I enjoyed reading this manuscript and have no comments that could potentially improve its quality. Response: Thank you for the kind words and encouragement. Reviewer #2: This well written paper on the effect of interpregnancy interval on labor dystocia presents data from the Michigan Inpatient Database. This database is a standard in the field and is known to be of good quality. The analytical approach is adequate, and the results are original. I have the following comments: 1. It is noted that 8.2% of the includible birth records (58,185/706,210) were excluded because the birth certificates had missing or implausible information on the infant's gestational age. Were there differences between these women excluded and those with complete data with regard to maternal age, parity, race, education, marital status, etc? Response: The following is a comparison of those with and those without missing data: With Missing Data Without Missing Data Mother's Race

White 74.8 77.6Black 20.0 19.4Native American 0.5 0.6Asian/Pacific Islander 2.7 2.0Other/Unknown 2.1 0.5

Education (years) 0-8 2.4 2.59-11 12.7 14.212 34.6 36.013-15 24.6 25.716+ 25.7 21.5

Mother's Age 10-19 5.1 3.920-24 19.0 21.925-29 28.7 30.130-34 29.4 28.935-39 14.6 12.840+ 3.2 2.3

Mother's marital status Married 69.8 71.1Other 30.2 28.9

Number of Previous Live Births 1 45.7 54.22 30.3 28.13 13.7 10.84+ 10.3 6.9

One can see that records associated with missing gestational age data were slightly less likely to be whites and more likely to be of unknown racial group; slightly more likely to be college graduates; tended to have extreme maternal ages (<20 or 35+ years); slightly less likely to be married; and more likely to be of higher parity. I did not include these data in the revised paper because the reviewer did not suggest that I include the data in the paper. However, should the Editor of the Journal or the reviewer find these data are important, I would be glad to include the data in the paper. 2. Why were the ICD-9 CM codes used to identify labor dystocia instead of the more recent ICD-10 CM codes? Response: I have included an explanation in the 2nd paragraph of the Materials and Methods section (p.4 of the cleaned manuscript). 3. Reference # 20 is incorrect. The correct reference is the following: Conde-Agudelo A, Belizan JM. Maternal morbidity and mortality associated with interpregnancy interval: cross sectional study. BMJ 2000;321:1255-9. Response: Thank you for catching this. I had pointed to the wrong entry in my Endnote library when I prepared the previous version of the paper. I have corrected this in this revised version. 4. More discussion should be included about the prevalence of labor dystocia found in the study. Is it high? Since one of the objectives of the study was to estimate the prevalence of labor dystocia, comparisons with data from other populations and countries are necessary. Response: Our paper is the first attempt (to our knowledge) to identify labor dystocia using the ICD-9 codes. There were no previously published population estimates of labor dystocia prevalence either in the U.S. or internationally. An article published on the eMedicine web site (http://www.emedicine.com/med/topic3280.htm) by Dr. Iraj Forouzan estimated that about 10% of deliveries in U.S. have labor dystocia, based on the reasoning that about 25-30% of deliveries are cesarean, and approximately 30%-40% of the cesarean deliveries are due to dystocia. Dr. Forouzan did not give any references for these data; we suspect this estimate is based on Dr. Forouzan’s clinical experience rather than actual population-based data. That said, we have addressed this point in the 3rd paragraph of the Comments section (pp.8-9 of the cleaned manuscript). 5. The authors should expand their discussion on the importance of maternal morbidities and other obstetrical variables that may be associated to both interpregnancy intervals and labor dystocia. For example, pre-pregnancy maternal obesity and diabetes, known risk factors for dystocia, could be more common among women with long interpregnancy intervals. These potential confounding factors might be taken into account in future studies. Response: We have addressed this point in the 7th paragraph of the Comments section (pp.10-11 of the cleaned manuscript.)

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Checklist for revised paper

1

Effect of the Interval Between Pregnancies on Labor Dystocia

Bao-Ping Zhu, MD, MS a

Violanda Grigorescu, MD, MSPH b

Thu Le, PhD b

Mei Lin, MD, MSc a

Glenn Copeland, MBA b

Maurice Barone b

George Turabelidze, MD, PhD a

From the Office of Epidemiology, Missouri Department of Health and Senior Services, a and

Division of Epidemiology Services, Michigan Department of Community Health. b

Correspondence Author: Bao-Ping Zhu, MD, MS, Office of Epidemiology, Missouri

Department of Health and Senior Services, 912 Wildwood Drive, P.O. Box 570, Jefferson City,

MO 65102, (573) 751-6128 (Tel); (573) 522-6003 (Fax); [email protected]

Word count: for abstract: 245; for text: 2855

1st Revised Manuscript - cleaned

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ABSTRACT

OBJECTIVE. The purposes of this study were to estimate the prevalence of labor dystocia, and

to evaluate its association with interpregnancy interval.

STUDY DESIGN. We linked the live birth certificate data for Michigan infants born from 1994

to 2002 with the hospital discharge data. A physician panel identified the International

Classification of Diseases (9th revision, clinical modifications, ICD-9-CM) codes indicating labor

dystocia, and classified the codes into functional and mechanical dystocia. We estimated the

prevalence of labor dystocia, and used stratified and logistic regression analyses to evaluate labor

dystocia in relation to interpregnancy interval, controlling for other reproductive risk factors.

RESULTS. Overall, 20.8% of the births experienced labor dystocia; 11.1% experienced

functional dystocia; and 12.5% experienced mechanical dystocia. Both functional and mechanical

dystocia were more prevalent in first births than in subsequent births; mechanical dystocia was

more prevalent in multiple births than in singleton births. In singleton births to multiparous

mothers, labor dystocia was linearly associated with the interpregnancy interval: For every year of

delayed conception, the risk for overall, functional, and mechanical dystocia increased by 3.6%

(95% confidence interval [CI], 3.3%-3.9%), 4.7% (95% CI, 4.3%-5.1%), 3.0% (95% CI, 2.6%-

3.3%), respectively, whereas the risk increase for individual constituents of labor dystocia varied

from 2.3% (for malposition or malpresentation) to 7.1% (for failed induction of labor), controlling

for other reproductive risk factors.

CONCLUSION. Labor dystocia is common among women during labor and delivery. In

singleton births to multiparous mothers, labor dystocia increased linearly with interpregnancy

interval.

Key words: interpregnancy interval; birth intervals, dystocia; difficult childbirth; difficult labor

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Labor dystocia, also known as “dysfunctional labor” or “difficult childbirth,” is common

among women during labor and delivery, and can have serious consequences (including maternal

deaths), especially in developing countries. For example, it is estimated that obstructed labor, a

component of labor dystocia, affects more than six million women globally, and accounts for 8%

(or 42,000) of the approximately half a million maternal deaths annually, mostly in developing

countries.1 Additionally, women who experience labor dystocia often require operative obstetric

procedures, including obstetric forceps, vacuum extraction, and cesarean delivery to assist with

the delivery. These operative procedures increase the risk for intracranial hemorrhage, peripheral

nerve injury, seizure, depressed 5-minute Apgar score, and assisted ventilation for the infant, as

well as lacerations, postpartum hemorrhage, thromboembolic events, anesthetic complications,

puerperal infection, obstetrical surgical wound infection for the mother.2-6 They can also cause

significant physical discomfort, psychological stress, and financial burden for the women, their

families, and the society.

A few studies have identified risk factors for obstructed labor (a constituent of labor

dystocia), including nutritional factors, underage childbearing, and poor access to reproductive

services.7 However, overall, the knowledge on the prevalence of labor dystocia as well as its

causes and risk factors is extremely scant. In this study we sought to identify labor dystocia using

the International Classification of Diseases, 9th revision, clinical modification (ICD-9-CM) codes.8

We used the livebirth certificate data for a large cohort of births linked to the delivery hospital

discharge data to estimate the prevalence of labor dystocia in the population. We also examined

labor dystocia in relation to interpregnancy interval, a potentially modifiable risk factor that has

been associated with various maternal and infant health outcomes.9-12

Material and Methods

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We linked the livebirth certificate data for births to Michigan resident women from January

1, 1994 to December 31, 2002 with the hospital discharge data for the same time period in the

Michigan Inpatient Database, using a deterministic approach. We used the mother’s medical

record number, date of birth, resident zip code, county of birth, and hospital of birth to perform

the linkage. The most stringent linkage criterion was that all variables matched. The criteria were

then gradually loosened, one variable at a time. The non-exact matches were verified using the

delivery date, admission date, discharge date, whether a cesarean delivery was performed, and

“diagnosis related groups” in the hospital discharge data. In a similar manner, we linked the birth

certificate data of infants born to the same biological mother, using the mother’s social security

number, birth date, first name, last name, middle initial, and maiden name on the birth certificate.

For non-exact matches, we used the mother’s address, the infant’s birth date, and other

information on the birth certificate to verify the links. (Detailed linkage methodology is available

from the corresponding author upon request.) The staff members at the Vital Records and Health

Data Development Section, Michigan Department of Community Health, who have statutory

authority to access identifying information, conducted the linkage. All identifying information was

removed from the linked dataset before statistical analyses were conducted.

A physician panel of an obstetrician, a pediatrician, and a medical epidemiologist identified

through consensus conditions indicating labor dystocia, based on the ICD-9-CM codes in the

Michigan Inpatient Database (Table I). We did not use the newer ICD-10 codes because the

clinical modification for classifying morbidities developed by the National Center for Health

Statistics was still in its pre-release format at the time this study was completed.13 These

conditions were classified into functional dystocia (including delayed delivery, failed induction,

uterine inertia or abnormal uterine contractions, prolonged labor) and mechanical dystocia

(including malposition or malpresentation of the fetus, obstructed labor, and disproportion). A

woman was classified as having labor dystocia if any of these conditions were among the

discharge diagnoses.

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We calculated the interpregnancy interval as the interval between the birth date of the index

infant and that of the preceding infant recorded on the birth certificate, minus the index infant’s

gestational age. Gestational age was calculated as the time between the date of the mother’s last

normal menstrual period and the infant’s date of birth, as recommended by the National Center for

Health Statistics.14 When the date of the last menstrual period was unavailable, we used the

clinically estimated gestational age recorded on the birth certificate. The interpregnancy interval

was computed in weeks and converted into months, assuming 13 weeks to be equal to three

months.

We evaluated the following risk factors as potential confounding factors: Infant’s birth

weight (<1500, 1500-2499, 2500-3999, and >4000g); mother’s age (categorized into five-year

age groups in the stratified analyses and used as a continuous variable in logistic regression), race,

marital status, educational attainment (<9, 9-11, 12, 13-15, and >16 years), total number of

previously recognized pregnancies, outcome of the preceding pregnancy (live birth or other

terminations), weight gain during pregnancy (categorized into quartiles), prenatal care utilization

(measured by the Kotelchuck Index15), smoking during pregnancy, and whether there was a

previous cesarean delivery (identified either by the ICD-9-CM code 654.2, or on the index

infant’s birth certificate). In the sensitivity analysis, when we used the infant’s gestational age

(<32, 32-36, 37-41, and >42 completed weeks) instead of birth weight, and the total number of

previous births instead of number of pregnancies, the results changed little (not shown).

We performed stratified analyses to examine the relationship between labor dystocia

(overall, functional, mechanical, and their constituents) and interpregnancy interval according to

the levels of the above-mentioned confounding factors, and used logistic regression16 to control

for the confounding factors simultaneously. When fitting the logistic regression model on

functional dystocia, we excluded records where mechanical dystocia was indicated; and vise

versa. Because the data for infants born to the same biological mother may be correlated, and this

correlation could result in biased variance estimates and incorrect statistical inferences if not

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appropriately accounted for,17 we used the generalized estimating equation (GEE) approach in the

logistic regression analyses with the exchangeable correlation structure.18 Also, we included

interpregnancy interval and mother’s age (in years) as continuous variables in the logistic

regression models because both interpregnancy interval and mother’s age were linearly associated

with labor dystocia. All other variables were included as categorical variables. In conducting the

logistic regression analyses with the GEE technique we used the statistical software package

SUDAAN (version 9.0; Research Triangle Institute, Research Triangle Park, NC); for all other

statistical analyses we used SAS (version 9.1; SAS Institute, Cary, NC).

The human subject review committees at the Michigan Department of Community Health,

the Michigan Hospital Association, and the Missouri Department of Health and Senior Services

independently approved this project.

Results

Between January 1, 1994 and December 31, 2002, a total of 1,210,757 live infants were

delivered to Michigan resident women. Hospital discharge records were identified in the Michigan

Inpatient Database for 1,180,211 (97.5%) of those births. Of those births with matched hospital

discharge data, 20.8% had ICD-9-CM codes indicating labor dystocia; 11.1% had functional

dystocia (2.2% had delayed delivery, 1.6% had failed induction, 7.4% had uterine inertia or

abnormal uterine contractions, and 1.1% had prolonged labor), and 12.5% had mechanical

dystocia (7.1% had malposition or malpresentation of the fetus, 5.8% had obstructed labor, and

2.4% had disproportion). (Of note, the percentages did not add up to the total percentages

because some women experienced multiple types of labor dystocia.) Labor dystocia was more

prevalent among first births (29.0% overall, 17.6% functional, and 16.5% mechanical) than

among subsequent births (15.4% overall, 6.9% functional, and 9.9% mechanical). Also, multiple

gestations were twice as likely to experience overall labor dystocia (42.0% vs. 20.1%) and three

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times as likely to experience mechanical dystocia (35.3% vs. 11.8%), but were slightly less likely

to experience functional dystocia (9.8% vs. 11.2%), compared with singleton births.

To evaluate the relationship between interpregnancy interval and labor dystocia, we

excluded 463,763 first births and 58,185 birth records for which the birth certificates had missing

or implausible information on the infant’s gestational age (hence rendering it impossible to

calculate the interpregnancy interval). In addition, we excluded 10,238 multiple births, leaving

648,025 singleton live births to multiparous women as the study population. The percentages of

overall labor dystocia (14.2%), functional dystocia (6.7%) and mechanical dystocia (8.8%) in this

study population were about two-thirds of those in the total population.

In examining the relationship between interpregnancy interval and labor dystocia, we found

labor dystocia (overall and both types) to be linearly associated with interpregnancy interval

(Figure 1). The individual constituents of labor dystocia were also linearly associated with

interpregnancy interval (Figure 2). When we stratified the data to examine the association

between labor dystocia and interpregnancy interval at levels of other reproductive risk factors, the

linear relationship between labor dystocia and interpregnancy interval existed in subgroups of

those risk factors where the data supported the stratified analyses (Table II). Specifically, to

evaluate whether the relationship was due to confounding by maternal age, which was correlated

with both interpregnancy interval and labor dystocia, we restricted the analysis to 30-year-old

mothers only and found the same linear relationship (not shown).

When we controlled for all potential confounding factors simultaneously using logistic

regression, the adjusted odds ratios associated with every 12 months of increment in the

interpregnancy interval was 1.0359 (95% CI, 1.0331-1.0387) for overall labor dystocia, 1.0469

(95% CI, 1.0431-1.0507) for functional dystocia, and 1.0297 (95% CI, 1.0263-1.0331) for

mechanical dystocia (Table III). Using these data, we estimated16 that for every year of delayed

conception following a live birth, the risk for overall, functional, and mechanical dystocia

increases by approximately 3.6%, 4.7%, and 3.0%, respectively. The increase in the risk would

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become substantial if the conception is delayed for a prolonged period of time. For example, a

five-year delay in conception would result in a 19% increase in overall dystocia, 26% increase in

functional dystocia, and 16% increase in mechanical dystocia, whereas a 10-year delay in

conception would result in risk increases of 42%, 58%, and 34%, respectively.

Comment

Labor dystocia, the opposite of labor eutocia or normal labor, can be caused by ineffective

expulsive forces of the uterine; an abnormal lie, presentation, position or fetal structure; or

disproportion between the sizes of the fetus and the pelvis, resulting in mechanical interferences

with the passage of the fetus through the birth canal. Every year, a large number of women

worldwide experience labor dystocia during labor and delivery, which can have dire

consequences, especially in developing countries.1 Yet little is known about the true prevalence of

labor dystocia and its various constituents in both developed and developing countries, and about

their related risk factors.

In this study, we assembled a large cohort of live births to Michigan women and linked the

data to the delivery discharge data in the Michigan Inpatient Database. We identified labor

dystocia based on the ICD-9-CM codes, and classified it into two categories: functional and

mechanical. We estimated that labor dystocia was experienced in approximately a fifth of all live

births in Michigan. When examining the two types of dystocia separately, both were substantially

more prevalent among first births than among subsequent births; mechanical dystocia, but not

functional dystocia, was more common among multiple births than among singleton births.

Among multiparous women who delivered a singleton infant, there was a linear association

between labor dystocia (overall, both types, and their individual constituents) and interpregnancy

interval. The association was stronger for functional dystocia than for mechanical dystocia. This

linear relationship persisted when the data were stratified by, and controlled for, other

reproductive risk factors.

9

To our knowledge, this study is the first attempt to measure labor dystocia in the general

population, and to evaluate the relationship between labor dystocia and interpregnancy interval.

Prior to our study, the information about the prevalence of labor dystocia had been extremely

scant both in the U.S. and internationally. The World Health Organization (WHO) estimated that,

worldwide, 4.6% of women giving births experience obstructed labor annually.1 This estimated

prevalence of obstructed labor, which was not based on population data, is about 20% lower than

the 5.8% estimated in our study. No information had been available from the WHO on other

constituents of labor dystocia. In the U.S., about 10% of labors were thought to have dystocia;19

this, too, was not a population estimate.

A number of studies have examined the relationship between interpregnancy interval and

various maternal and infant health outcomes. A short interpregnancy interval has been associated

with various adverse health outcomes for the mother, including uterine rupture,20, 21 third trimester

bleeding, premature rupture of membranes, puerperal endometritis, and maternal death,22 whereas

a long interpregnancy interval has been associated with preeclampsia and eclampsia.9, 22 In

addition, both short and long interpregnancy intervals have been associated with adverse health

outcomes for the newborn, including low birthweight, preterm birth, and small size for gestational

age.10-12 Regarding the possible biological mechanisms, the effect of a short interpregnancy

interval on adverse health outcomes for mother and infant is thought to be due to maternal

nutritional depletion and postpartum physiological and psychological stress.23, 24 However, the

effect of a long interpregnancy interval is poorly understood. A recently proposed hypothesis

theorized that pregnancy may physiologically prepare and optimize the growth-supporting

capacities of the mother. After delivery, the mother may gradually lose those capacities and

physiologically become similar to a primigravida woman if another pregnancy is not timely

conceived.12 This hypothesis, known as the “physiological regression hypothesis,” appears to be

consistent with the findings of the current study: As the interpregnancy interval increases, the

childrearing capacities developed during the preceding pregnancy (e.g., primed hormonal profile,

10

improved uterine muscle functions, and enhanced pelvis architecture) may decline, leading to

labor dystocia.

Although out of the scope of this paper, it was interesting to notice the association between

labor dystocia and many of the reproductive risk factors measured in this study. Further studies

are needed to examine those relationships.

This study has three major strengths: First, it assembled a large number of births, which

enabled detailed analyses on the potential confounding effects of many reproductive risk factors,

especially maternal age. Second, we linked the birth data with the hospital discharge data, greatly

enriching the dataset with the ICD-9-CM diagnosis codes not available in the birth file alone.

Third, we linked the infants to their biological mothers, allowing us to use the GEE technique to

account for the correlation among biological siblings.

However, this study also has several limitations, which should be noted in interpreting its

findings. First, in estimating the interpregnancy interval we used the preceding infant’s birth date

and the date of the mother’s last menstrual period recorded on the birth certificate, both of which

may be subject to errors. To evaluate the accuracy of the preceding infant’s birth date reported by

the mother, we used the maternally linked birth data created in a previous study11 to calculate the

interpregnancy interval, based on the birth dates of two consecutive live births recorded on the

birth certificates. We found the interpregnancy interval computed in this manner to be virtually

identical to that based on the previous infant’s birth date reported by the mother (means: 26.52 vs.

26.55; correlation coefficient: 0.999, p<0.0001). Likewise, although gestational age estimated

from the last menstrual period may be subject to various errors,25 the errors (mostly no more than

a few weeks) are likely to be small relative to the length of the interpregnancy interval; hence we

expect these errors to have minimal impact on the results of this study. Second, no hospital

discharge data were found for approximately 2.4% of births, many of which are likely to be home

births. These births are probably at lower risk for labor dystocia. However, since this percentage

is small, we expect the resultant bias, if any, to be minimal. Third, this study used a linked birth

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and hospital discharge database, which have been shown to provide accurate estimates for certain

obstetric conditions.26 However, whether labor dystocia can be accurately identified using this

dataset is unknown, although the use of multiple ICD-9-CM codes to identify labor dystocia may

have helped to reduce the underestimation. Fourth, due to data limitations we were unable to

assess maternal morbidities and other obstetrical variables that may be associated with both

interpregnancy interval and labor dystocia. For example, pre-pregnancy maternal obesity and

diabetes, a known risk factors for cesarean delivery and likely also for labor dystocia, may be

more common among women with long interpregnancy intervals. Future studies should

investigate whether these maternal morbidities may confound the relationship between labor

dystocia and interpregnancy interval, and whether the confounding is mediated through maternal

age. Fifth, this study is based on the U.S. data, which may not be generalizable to developing

countries, where labor dystocia has the most serious consequences. For these reasons this study

needs to be interpreted with caution, and replicated in future studies in other settings, especially

developing countries.

The findings of this study, if corroborated by other studies, could be useful for counseling

postpartum women who are planning for another pregnancy about the risk of prolonged

pregnancy spacing for labor dystocia. Obstetric healthcare providers can also use the information

to more accurately assess a woman’s risk for labor abnormality and labor dystocia at hospital

admission, and to develop a more effective labor management plan.

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21. Bujold E, Mehta SH, Bujold C, Gauthier RJ. Interdelivery interval and uterine rupture. Am J Obstet Gynecol 2002;187:1199-202.

22. Conde-Agudelo A, Belizan JM. Maternal morbidity and mortality associated with interpregnancy interval: cross sectional study. BMJ 2000;321:1255-9.

23. Miller JE. Birth intervals and perinatal health: an investigation of three hypotheses. Fam Plann Perspect 1991;23:62-70.

24. Winkvist A, Rasmussen KM, Habicht JP. A new definition of maternal depletion syndrome. Am J Public Health 1992;82:691-4.

25. Alexander GR, Tompkins ME, Petersen DJ, Hulsey TC, Mor J. Discordance between LMP-based and clinically estimated gestational age: implications for research, programs, and policy. Public Health Rep 1995;110:395-402.

26. Parrish KM, Holt VL, Connell FA, Williams B, LoGerfo JP. Variations in the accuracy of obstetric procedures and diagnoses on birth records in Washington State, 1989. Am J Epidemiol 1993;138:119-27.

14

Table I. Diagnosis and Procedure Codes Indicating Labor Dystocia, Based on the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) Functional Dystocia Delayed delivery 658.2 Delayed delivery after spontaneous/unspecified rupture of membranes 658.3 Delayed delivery after artificial rupture of membranes Failed induction 659.0 Failed mechanical induction (surgical/other instrument methods) 659.1 Failed medical/unspecified induction (e.g. oxytocin) 660.6 Failed trial of labor, unspecified 660.7 Failed forceps or vacuum extractor, unspecified Uterine inertia or abnormal uterine contractions 661.0 Primary uterine inertia 661.1 Secondary uterine inertia 661.2 Other and unspecified uterine inertia 661.4 Hypertonic, incoordinate, or prolonged uterine contractions 763.7 Abnormal uterine contractions Prolonged labor 662.0 Prolonged first stage 662.1 Prolonged labor, unspecified 662.2 Prolonged second stage Mechanical Dystocia Malposition or malpresentation of fetus 652.0 Unstable lie 652.1 Breech or other malpresentation successfully converted to cephalic presentation 652.2 Breech presentation without mention of version 652.3 Transverse or oblique presentation 652.4 Face or brow presentation 652.5 High head at term 652.7 Prolapsed arm 652.8 Other specified malposition or malpresentation 652.9 Unspecified malposition or malpresentation Obstructed labor 660.0 Obstruction caused by malposition of fetus at onset of labor 660.1 Obstruction by bony pelvis 660.2 Obstruction by abnormal pelvic soft tissues 660.3 Deep transverse arrest and persistent occipitoposterior position 660.4 Shoulder (girdle) dystocia 660.8 Other causes of obstructed labor 660.9 Unspecified obstructed labor Disproportion 653.0 Major abnormality of bony pelvis, not further specified 653.1 Generally contracted pelvis 653.2 Inlet contraction of pelvis 653.3 Outlet contraction of pelvis 653.4 Fetopelvic disproportion 653.5 Unusually large fetus causing disproportion 653.6 Hydrocephalic fetus causing disproportion 653.7 Other fetal abnormality causing disproportion

653.8 Disproportion of other origin; excluding shoulder (girdle) dystocia 653.9 Unspecified disproportion

15

Table II. Percent of labor dystocia by interpregnancy interval according to levels of selected reproductive risk factors: Singleton live births to multiparous mothers, Michigan, 1994-2002 Interpregnancy Interval (months)

0-11 12-23 24-35 36-47 48-59 60-71 72-83 84-95 96-107 108-119 120+

Overall 12.5 13.2 13.8 14.4 14.9 15.6 16.4 17.1 17.6 17.7 20.4Infant birthweight (g)

<1500 45.8 40.6 41.1 45.1 43.2 44.1 48.1 43.5 46.5 48.8 47.21500-<2500 18.3 20.5 20.9 21.4 21.0 21.4 22.1 24.2 21.6 22.3 24.22500-<4000 10.6 11.3 12.0 12.5 12.8 13.7 13.9 14.7 15.4 15.5 17.84000+ 20.1 21.1 21.4 22.4 24.0 23.5 27.2 26.8 27.8 26.7 31.5

Mother’s age (yrs) 10-19 11.3 11.7 11.4 14.8 15.5 11.4 18.8 50.0 . . .20-24 11.4 12.4 12.7 13.2 13.8 15.3 15.4 15.4 17.4 17.9 23.125-29 12.3 13.1 13.8 14.2 14.6 15.0 15.6 16.5 17.0 16.6 18.630-34 13.7 13.7 14.3 15.1 15.4 16.1 16.2 17.4 18.1 17.5 19.435-39 15.5 14.8 15.0 15.5 15.7 16.1 18.1 17.7 18.1 18.1 20.740+ 17.2 15.5 17.3 15.7 18.0 19.2 19.0 19.1 15.8 20.4 22.5

Mother's race White 12.9 13.4 14.0 14.5 15.0 15.6 16.3 17.1 17.6 17.8 20.1Black 11.2 12.3 13.1 14.0 14.6 15.6 16.5 17.5 17.9 16.7 20.9Native American 12.7 13.8 16.3 13.9 19.8 23.7 18.2 16.8 7.6 18.4 27.5Asian/Pacific Islander 11.5 12.4 13.7 15.6 14.4 16.0 16.1 12.3 18.2 19.9 22.4Other/Unknown 14.5 14.2 16.0 20.8 17.4 15.2 20.5 21.1 14.9 38.9 22.4

Mother's marital status Married 13.1 13.6 14.2 14.8 15.2 15.7 16.6 17.4 18.0 18.5 20.5Other 11.5 12.1 12.6 13.6 14.4 15.5 16.0 16.7 17.0 16.1 20.3

Mother's education (yrs) 0-8 11.7 12.5 12.8 13.8 13.0 17.0 13.9 18.0 17.5 21.9 22.19-11 11.3 12.1 12.4 13.5 14.1 15.1 14.3 15.2 16.3 16.2 20.312 12.1 13.1 13.6 14.5 14.7 15.2 16.3 17.0 17.9 17.4 20.313-15 13.0 13.8 14.4 14.5 15.5 16.0 16.8 17.8 17.4 17.3 20.316+ 14.1 13.5 14.4 14.8 15.1 16.1 17.3 17.3 18.5 19.1 20.9

Number of previous pregnancies 1 13.9 14.0 14.9 15.7 16.0 16.6 16.8 18.4 19.7 18.3 21.02 11.8 12.9 13.0 13.8 14.2 14.8 16.6 16.6 16.0 18.0 19.93 11.3 12.4 12.9 13.7 13.9 15.2 16.0 15.3 16.7 15.0 20.14 11.3 12.1 13.7 12.8 14.1 16.4 14.4 17.2 17.0 18.5 20.25+ 12.1 12.7 13.0 14.6 16.0 15.4 16.9 18.5 18.4 19.2 20.7

Outcome of last pregnancy Live birth 12.5 13.2 13.8 14.4 14.8 15.5 16.2 17.0 17.7 17.4 20.2Other terminations 13.1 13.8 14.4 14.6 15.3 16.2 16.8 17.6 17.5 18.8 21.1

Weight gain during pregnancy (kg) 0-9.4 11.9 13.1 13.6 14.5 14.8 15.3 16.6 17.5 17.1 17.3 19.69.5-13.5 11.9 12.6 13.1 13.5 14.3 14.8 15.1 16.7 16.7 17.1 20.013.5-18.0 12.6 13.0 13.9 14.1 14.3 15.4 15.5 16.5 16.9 17.4 20.0>18 13.8 14.2 14.8 15.6 15.9 16.4 17.4 16.9 19.1 18.9 21.8

16

Table II cont’d. Kotelchuck Prenatal Care Index

Adequate plus 13.6 14.3 14.8 15.6 15.9 17.0 16.3 18.8 17.9 18.9 21.6Adequate 12.5 12.8 13.2 13.8 14.5 14.7 16.2 15.9 17.2 17.5 19.7Intermediate 12.0 13.2 14.1 15.1 14.3 15.2 16.8 17.6 17.1 16.4 20.2Inadequate 11.5 12.3 13.5 13.2 14.3 15.9 16.7 15.5 18.5 14.5 19.0Unknown 11.3 12.6 14.2 13.6 14.2 14.8 17.1 17.0 18.9 18.4 20.1

Smoking during pregnancy Yes 11.9 12.3 12.8 13.2 14.6 15.1 14.8 16.1 15.7 16.2 19.5No 12.6 13.4 14.0 14.7 15.0 15.8 16.8 17.5 18.2 18.1 20.8Unknown 14.2 15.9 15.4 17.9 14.3 17.5 22.4 14.9 23.2 24.0 20.3

Previous cesarean delivery No 11.2 11.9 12.5 13.1 13.7 14.4 15.2 16.5 16.7 16.9 19.9Yes 19.1 19.9 20.1 20.2 20.2 21.2 21.3 19.8 21.7 21.2 22.8

17

Table III. Adjusted odds ratios* for labor dystocia (overall, functional, mechanical, and their constituents) associated with every 12 months of increment in interpregnancy interval: Matched birth-hospital discharge data, Michigan, 1994-2002.

Adjusted Odds Ratio 95% Confidence Interval

Labor Dystocia 1.0359 1.0331 - 1.0387

Functional Dystocia 1.0469 1.0431 - 1.0507 Delayed delivery 1.0396 1.0319 - 1.0473 Failed induction 1.0708 1.0615 - 1.0801 Uterine inertia or abnormal uterine contractions 1.0487 1.0440 - 1.0534 Prolonged labor 1.0401 1.0256 - 1.0547

Mechanical Dystocia 1.0297 1.0263 - 1.0331

Malposition or malpresentation of fetus 1.0231 1.0188 - 1.0275 Obstructed labor 1.0367 1.0318 - 1.0416 Disproportion 1.0513 1.0423 - 1.0605 * Controlled for infant’s birth weight (<1500, 1500-2499, 2500-3999, 4000+g); mother’s age, race, marital status, education (<9, 9-11, 12, 13-15, 16+ years), total number of previously recognized pregnancies, outcome of the preceding pregnancy (live birth or other terminations), weight gain during pregnancy, prenatal care utilization, smoking during pregnancy, and previous cesarean delivery.

0.0

5.0

10.0

15.0

20.0

25.0

0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102 108 114 120 126 132 138 144 150

Interpregnancy Interval (mos)

%

Labor Dystocia Functional Dystocia Mechanical Dystocia

Figure1

Figure 1. Interpregnancy interval in relation to labor dystocia (overall, functional, and mechanical): Linked birth and hospital discharge data, Michigan, 1994-2002

Figure1 title

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102 108 114 120 126 132 138 144 150Interpregnancy Interval (mos)

%

Delayed Delivery Failed Induction

Uterine inertia / abnormal uterine contractions Prolonged laborMalposition or malpresentation of fetus Obstructed labor

Disproportion

Figure2

Figure 2. Interpregnancy interval in relation to individual constituents of labor dystocia: Linked birth and hospital discharge data, Michigan, 1994-2002

Figure2 title

1




Thu Le, PhD b

Mei Lin, MD, MSc a

Glenn Copeland, MBA b

Maurice Barone b







Word count: for abstract: 245; for text: 2855Deleted: 2628

Red-lined edited manuscript - showing changes

2

ABSTRACT




to 2002 with the hospital discharge data. A physician panel identified the International

Classification of Diseases (9th revision, clinical modifications, ICD-9-CM) codes indicating labor

dystocia, and classified the codes into functional and mechanical dystocia. We estimated the

prevalence of labor dystocia, and used stratified and logistic regression analyses to evaluate labor

dystocia in relation to interpregnancy interval, controlling for other reproductive risk factors.

RESULTS. Overall, 20.8% of the births experienced labor dystocia; 11.1% experienced

functional dystocia; and 12.5% experienced mechanical dystocia. Both functional and mechanical

dystocia were more prevalent in first births than in subsequent births; mechanical dystocia was

more prevalent in multiple births than in singleton births. In singleton births to multiparous

mothers, labor dystocia was linearly associated with the interpregnancy interval: For every year of

delayed conception, the risk for overall, functional, and mechanical dystocia increased by 3.6%

(95% confidence interval [CI], 3.3%-3.9%), 4.7% (95% CI, 4.3%-5.1%), 3.0% (95% CI, 2.6%-

3.3%), respectively, whereas the risk increase for individual constituents of labor dystocia varied

from 2.3% (for malposition or malpresentation) to 7.1% (for failed induction of labor), controlling

for other reproductive risk factors.



interval.

Key words: interpregnancy interval; birth intervals, dystocia; difficult childbirth; difficult labor

Deleted: for this study

Deleted: Labor dystocia was experienced in

Deleted: (

Deleted: ,

Deleted: )

Deleted: higher-order

Deleted: related

Deleted: The risk increase for the individual constituents of labor dystocia associated with every year of delayed conception varied from 2.3% (for malposition or malpresentation) to 7.1% (for failed induction of labor).

3
















services.7 However, overall, the knowledge on the prevalence of labor dystocia as well as its

causes and risk factors is extremely scant. In this study we sought to identify labor dystocia using

the International Classification of Diseases, 9th revision, clinical modification (ICD-9-CM) codes.8

We used the livebirth certificate data for a large cohort of births linked to the delivery hospital





Deleted: about

Deleted: ,

4



















Michigan Inpatient Database (Table I). We did not use the newer ICD-10 codes because the

clinical modification for classifying morbidities developed by the National Center for Health

Statistics was still in its pre-release format at the time this study was completed.13 These

conditions were classified into functional dystocia (including delayed delivery, failed induction,

uterine inertia or abnormal uterine contractions, prolonged labor) and mechanical dystocia

(including malposition or malpresentation of the fetus, obstructed labor, and disproportion). A

woman was classified as having labor dystocia if any of these conditions were among the

discharge diagnoses.

Deleted: 1

Deleted: and

Deleted: and

5








months.

We evaluated the following risk factors as potential confounding factors: Infant’s birth

weight (<1500, 1500-2499, 2500-3999, and >4000g); mother’s age (categorized into five-year

age groups in the stratified analyses and used as a continuous variable in logistic regression), race,






infant’s birth certificate). In the sensitivity analysis, when we used the infant’s gestational age

(<32, 32-36, 37-41, and >42 completed weeks) instead of birth weight, and the total number of

previous births instead of number of pregnancies, the results changed little (not shown).

We performed stratified analyses to examine the relationship between labor dystocia

(overall, functional, mechanical, and their constituents) and interpregnancy interval according to

the levels of the above-mentioned confounding factors, and used logistic regression16 to control

for the confounding factors simultaneously. When fitting the logistic regression model on

functional dystocia, we excluded records where mechanical dystocia was indicated; and vise

versa. Because the data for infants born to the same biological mother may be correlated, and this

correlation could result in biased variance estimates and incorrect statistical inferences if not

Deleted: Also, i

Deleted: the results changed little

Deleted: of

6

appropriately accounted for,17 we used the generalized estimating equation (GEE) approach in the

logistic regression analyses with the exchangeable correlation structure.18 Also, we included










Results

Between January 1, 1994 and December 31, 2002, a total of 1,210,757 live infants were

delivered to Michigan resident women. Hospital discharge records were identified in the Michigan

Inpatient Database for 1,180,211 (97.5%) of those births. Of those births with matched hospital

discharge data, 20.8% had ICD-9-CM codes indicating labor dystocia; 11.1% had functional

dystocia (2.2% had delayed delivery, 1.6% had failed induction, 7.4% had uterine inertia or

abnormal uterine contractions, and 1.1% had prolonged labor), and 12.5% had mechanical

dystocia (7.1% had malposition or malpresentation of the fetus, 5.8% had obstructed labor, and

2.4% had disproportion). (Of note, the percentages did not add up to the total percentages

because some women experienced multiple types of labor dystocia.) Labor dystocia was more

prevalent among first births (29.0% overall, 17.6% functional, and 16.5% mechanical) than

among subsequent births (15.4% overall, 6.9% functional, and 9.9% mechanical). Also, multiple

gestations were twice as likely to experience overall labor dystocia (42.0% vs. 20.1%) and three

Deleted: 1,181,847

Deleted: 6

Deleted: 16.0

Deleted: of the two types of labor dystocia

Deleted: of overall labor dystocia

Deleted: had

Deleted: both


7

times as likely to experience mechanical dystocia (35.3% vs. 11.8%), but were slightly less likely

to experience functional dystocia (9.8% vs. 11.2%), compared with singleton births.














those risk factors where the data supported the stratified analyses (Table II). Specifically, to


with both interpregnancy interval and labor dystocia, we restricted the analysis to 30-year-old

mothers only and found the same linear relationship (not shown).


regression, the adjusted odds ratios associated with every 12 months of increment in the



mechanical dystocia (Table III). Using these data, we estimated16 that for every year of delayed

conception following a live birth, the risk for overall, functional, and mechanical dystocia

increases by approximately 3.6%, 4.7%, and 3.0%, respectively. The increase in the risk would Formatted

Deleted: 2

Deleted: only

Deleted: (aORs)

Deleted: 3

Deleted: These

Deleted: indicate

8

become substantial if the conception is delayed for a prolonged period of time. For example, a

five-year delay in conception would result in a 19% increase in overall dystocia, 26% increase in

functional dystocia, and 16% increase in mechanical dystocia, whereas a 10-year delay in

conception would result in risk increases of 42%, 58%, and 34%, respectively.

Comment

Labor dystocia, the opposite of labor eutocia or normal labor, can be caused by ineffective



with the passage of the fetus through the birth canal. Every year, a large number of women

worldwide experience labor dystocia during labor and delivery, which can have dire

consequences, especially in developing countries.1 Yet little is known about the true prevalence of

labor dystocia and its various constituents in both developed and developing countries, and about

their related risk factors.




mechanical. We estimated that labor dystocia was experienced in approximately a fifth of all live


more prevalent among first births than among subsequent births; mechanical dystocia, but not

functional dystocia, was more common among multiple births than among singleton births.





reproductive risk factors.

Deleted: ,

Deleted: have

Deleted: especially in developing countries,

Deleted: there is insufficient

Deleted: information about the magnitude of the problem

Deleted: its

Deleted: by


Deleted: were

9

To our knowledge, this study is the first attempt to measure labor dystocia in the general

population, and to evaluate the relationship between labor dystocia and interpregnancy interval.

Prior to our study, the information about the prevalence of labor dystocia had been extremely

scant both in the U.S. and internationally. The World Health Organization (WHO) estimated that,

worldwide, 4.6% of women giving births experience obstructed labor annually.1 This estimated

prevalence of obstructed labor, which was not based on population data, is about 20% lower than

the 5.8% estimated in our study. No information had been available from the WHO on other

constituents of labor dystocia. In the U.S., about 10% of labors were thought to have dystocia;19

this, too, was not a population estimate.





a long interpregnancy interval has been associated with preeclampsia and eclampsia.9, 22 In







theorized that pregnancy may physiologically prepare and optimize the growth-supporting




consistent with the findings of the current study: As the interpregnancy interval increases, the


Deleted: theorizes

Deleted: results

10


labor dystocia.








Third, we linked the infants to their biological mothers, allowing us to use the GEE technique to

account for the correlation among biological siblings.




may be subject to errors. To evaluate the accuracy of the preceding infant’s birth date reported by





26.55; correlation coefficient: 0.999, p<0.0001). Likewise, although gestational age estimated

from the last menstrual period may be subject to various errors,25 the errors (mostly no more than

a few weeks) are likely to be small relative to the length of the interpregnancy interval; hence we

expect these errors to have minimal impact on the results of this study. Second, no hospital

discharge data were found for approximately 2.4% of births, many of which are likely to be home

births. These births are probably at lower risk for labor dystocia. However, since this percentage

is small, we expect the resultant bias, if any, to be minimal. Third, this study used a linked birth

Deleted: perform

Deleted: analysis

Deleted: ing

Deleted: Also

Deleted: .

Deleted: However, since

Deleted: in estimated gestational age

Deleted: less

Deleted: ,

Deleted:

11

and hospital discharge database, which have been shown to provide accurate estimates for certain

obstetric conditions.26 However, whether labor dystocia can be accurately identified using this

dataset is unknown, although the use of multiple ICD-9-CM codes to identify labor dystocia may

have helped to reduce the underestimation. Fourth, due to data limitations we were unable to

assess maternal morbidities and other obstetrical variables that may be associated with both

interpregnancy interval and labor dystocia. For example, pre-pregnancy maternal obesity and

diabetes, a known risk factors for cesarean delivery and likely also for labor dystocia, may be

more common among women with long interpregnancy intervals. Future studies should

investigate whether these maternal morbidities may confound the relationship between labor

dystocia and interpregnancy interval, and whether the confounding is mediated through maternal

age. Fifth, this study is based on the U.S. data, which may not be generalizable to developing

countries, where labor dystocia has the most serious consequences. For these reasons this study

needs to be interpreted with caution, and replicated in future studies in other settings, especially

developing countries.






Deleted: has been

Deleted: in this study

Deleted:

Deleted: Although out of the scope of this paper, it was interesting to notice the association between labor dystocia and many of the reproductive risk factors measured in this study. Further studies are needed to examine those relationships.¶

12

REFERENCES


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3. Towner D, Castro MA, Eby-Wilkens E, Gilbert WM. Effect of mode of delivery in nulliparous women on neonatal intracranial injury. N Engl J Med 1999;341:1709-14.

4. Johnson JH, Figueroa R, Garry D, Elimian A, Maulik D. Immediate maternal and neonatal effects of forceps and vacuum-assisted deliveries. Obstet Gynecol 2004;103:513-8.

5. Koroukian SM. Relative risk of postpartum complications in the Ohio Medicaid population: vaginal versus cesarean delivery. Med Care Res Rev 2004;61:203-24.

6. Wen SW, Liu S, Kramer MS, Marcoux S, Ohlsson A, Sauve R, et al. Comparison of maternal and infant outcomes between vacuum extraction and forceps deliveries. Am J Epidemiol 2001;153:103-7.

7. Konje JC, Ladipo OA. Nutrition and obstructed labor. Am J Clin Nutr 2000;72:291S-7S.


9. Skjaerven R, Wilcox AJ, Lie RT. The interval between pregnancies and the risk of preeclampsia. N Engl J Med 2002;346:33-8.

10. Zhu BP, Haines KM, Le T, McGrath-Miller K, Boulton ML. Effect of the interval between pregnancies on perinatal outcomes among white and black women. Am J Obstet Gynecol 2001;185:1403-10.

11. Zhu BP, Le T. Effect of interpregnancy interval on infant low birth weight: a retrospective cohort study using the Michigan Maternally Linked Birth Database. Matern Child Health J 2003;7:169-78.

12. Zhu BP, Rolfs RT, Nangle BE, Horan JM. Effect of the interval between pregnancies on perinatal outcomes. N Engl J Med 1999;340:589-94.

13. Pre-release Draft, June 2003: International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM). 2003. (Accessed October 14, 2005, at http://www.cdc.gov/nchs/about/otheract/icd9/icd10cm.htm.)

14. Ventura SJ, Martin JA, Curtin SC, Mathews TJ. Report of final natality statistics, 1995. Mon Vital Stat Rep 1997;45 (Suppl).

15. Kotelchuck M. The Adequacy of Prenatal Care Utilization Index: its US distribution and

13

association with low birthweight. Am J Public Health 1994;84:1486-9.


17. Watier L, Richardson S, Hemon D. Accounting for pregnancy dependence in epidemiologic studies of reproductive outcomes. Epidemiology 1997;8:629-36.

18. Zeger SL, Liang KY, Albert PS. Models for longitudinal data: a generalized estimating equation approach. Biometrics 1988;44:1049-60.

19. Dystocia. eMedicine, 2004. (Accessed Ocbober 14, 2005, at http://www.emedicine.com/med/topic3280.htm.)

20. Shipp TD, Zelop CM, Repke JT, Cohen A, Lieberman E. Interdelivery interval and risk of symptomatic uterine rupture. Obstet Gynecol 2001;97:175-7.

21. Bujold E, Mehta SH, Bujold C, Gauthier RJ. Interdelivery interval and uterine rupture. Am J Obstet Gynecol 2002;187:1199-202.

22. Conde-Agudelo A, Belizan JM. Maternal morbidity and mortality associated with interpregnancy interval: cross sectional study. BMJ 2000;321:1255-9.

23. Miller JE. Birth intervals and perinatal health: an investigation of three hypotheses. Fam Plann Perspect 1991;23:62-70.

24. Winkvist A, Rasmussen KM, Habicht JP. A new definition of maternal depletion syndrome. Am J Public Health 1992;82:691-4.

25. Alexander GR, Tompkins ME, Petersen DJ, Hulsey TC, Mor J. Discordance between LMP-based and clinically estimated gestational age: implications for research, programs, and policy. Public Health Rep 1995;110:395-402.

26. Parrish KM, Holt VL, Connell FA, Williams B, LoGerfo JP. Variations in the accuracy of obstetric procedures and diagnoses on birth records in Washington State, 1989. Am J Epidemiol 1993;138:119-27.

Formatted

14

Table I. Diagnosis and Procedure Codes Indicating Labor Dystocia, Based on the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) Functional Dystocia Delayed delivery 658.2 Delayed delivery after spontaneous/unspecified rupture of membranes 658.3 Delayed delivery after artificial rupture of membranes Failed induction 659.0 Failed mechanical induction (surgical/other instrument methods) 659.1 Failed medical/unspecified induction (e.g. oxytocin) 660.6 Failed trial of labor, unspecified 660.7 Failed forceps or vacuum extractor, unspecified Uterine inertia or abnormal uterine contractions 661.0 Primary uterine inertia 661.1 Secondary uterine inertia 661.2 Other and unspecified uterine inertia 661.4 Hypertonic, incoordinate, or prolonged uterine contractions 763.7 Abnormal uterine contractions Prolonged labor 662.0 Prolonged first stage 662.1 Prolonged labor, unspecified 662.2 Prolonged second stage Mechanical Dystocia Malposition or malpresentation of fetus 652.0 Unstable lie 652.1 Breech or other malpresentation successfully converted to cephalic presentation 652.2 Breech presentation without mention of version 652.3 Transverse or oblique presentation 652.4 Face or brow presentation 652.5 High head at term 652.7 Prolapsed arm 652.8 Other specified malposition or malpresentation 652.9 Unspecified malposition or malpresentation Obstructed labor 660.0 Obstruction caused by malposition of fetus at onset of labor 660.1 Obstruction by bony pelvis 660.2 Obstruction by abnormal pelvic soft tissues 660.3 Deep transverse arrest and persistent occipitoposterior position 660.4 Shoulder (girdle) dystocia 660.8 Other causes of obstructed labor 660.9 Unspecified obstructed labor Disproportion 653.0 Major abnormality of bony pelvis, not further specified 653.1 Generally contracted pelvis 653.2 Inlet contraction of pelvis 653.3 Outlet contraction of pelvis 653.4 Fetopelvic disproportion 653.5 Unusually large fetus causing disproportion 653.6 Hydrocephalic fetus causing disproportion 653.7 Other fetal abnormality causing disproportion


Deleted: 1

Deleted: del

Deleted: and

Deleted: and

15

Table II. Percent of labor dystocia by interpregnancy interval according to levels of selected reproductive risk factors: Singleton live births to multiparous mothers, Michigan, 1994-2002 Interpregnancy Interval (months) 0-11 12-23 24-35 36-47 48-59 60-71 72-83 84-95 96-107 108-119 120+ Overall 12.5 13.2 13.8 14.4 14.9 15.6 16.4 17.1 17.6 17.7 20.4 Infant birthweight (g)

<1500 45.8 40.6 41.1 45.1 43.2 44.1 48.1 43.5 46.5 48.8 47.2 1500-<2500 18.3 20.5 20.9 21.4 21.0 21.4 22.1 24.2 21.6 22.3 24.2 2500-<4000 10.6 11.3 12.0 12.5 12.8 13.7 13.9 14.7 15.4 15.5 17.8 4000+ 20.1 21.1 21.4 22.4 24.0 23.5 27.2 26.8 27.8 26.7 31.5

Mother’s age (yrs) 10-19 11.3 11.7 11.4 14.8 15.5 11.4 18.8 50.0 . . . 20-24 11.4 12.4 12.7 13.2 13.8 15.3 15.4 15.4 17.4 17.9 23.1 25-29 12.3 13.1 13.8 14.2 14.6 15.0 15.6 16.5 17.0 16.6 18.6 30-34 13.7 13.7 14.3 15.1 15.4 16.1 16.2 17.4 18.1 17.5 19.4 35-39 15.5 14.8 15.0 15.5 15.7 16.1 18.1 17.7 18.1 18.1 20.7 40+ 17.2 15.5 17.3 15.7 18.0 19.2 19.0 19.1 15.8 20.4 22.5

Mother's race White 12.9 13.4 14.0 14.5 15.0 15.6 16.3 17.1 17.6 17.8 20.1 Black 11.2 12.3 13.1 14.0 14.6 15.6 16.5 17.5 17.9 16.7 20.9 Native American 12.7 13.8 16.3 13.9 19.8 23.7 18.2 16.8 7.6 18.4 27.5 Asian/Pacific Islander 11.5 12.4 13.7 15.6 14.4 16.0 16.1 12.3 18.2 19.9 22.4 Other/Unknown 14.5 14.2 16.0 20.8 17.4 15.2 20.5 21.1 14.9 38.9 22.4

Mother's marital status Married 13.1 13.6 14.2 14.8 15.2 15.7 16.6 17.4 18.0 18.5 20.5 Other 11.5 12.1 12.6 13.6 14.4 15.5 16.0 16.7 17.0 16.1 20.3

Mother's education (yrs) 0-8 11.7 12.5 12.8 13.8 13.0 17.0 13.9 18.0 17.5 21.9 22.1 9-11 11.3 12.1 12.4 13.5 14.1 15.1 14.3 15.2 16.3 16.2 20.3 12 12.1 13.1 13.6 14.5 14.7 15.2 16.3 17.0 17.9 17.4 20.3 13-15 13.0 13.8 14.4 14.5 15.5 16.0 16.8 17.8 17.4 17.3 20.3 16+ 14.1 13.5 14.4 14.8 15.1 16.1 17.3 17.3 18.5 19.1 20.9

Number of previous pregnancies 1 13.9 14.0 14.9 15.7 16.0 16.6 16.8 18.4 19.7 18.3 21.0 2 11.8 12.9 13.0 13.8 14.2 14.8 16.6 16.6 16.0 18.0 19.9 3 11.3 12.4 12.9 13.7 13.9 15.2 16.0 15.3 16.7 15.0 20.1 4 11.3 12.1 13.7 12.8 14.1 16.4 14.4 17.2 17.0 18.5 20.2 5+ 12.1 12.7 13.0 14.6 16.0 15.4 16.9 18.5 18.4 19.2 20.7

Outcome of last pregnancy Live birth 12.5 13.2 13.8 14.4 14.8 15.5 16.2 17.0 17.7 17.4 20.2 Other terminations 13.1 13.8 14.4 14.6 15.3 16.2 16.8 17.6 17.5 18.8 21.1

Weight gain during pregnancy (kg) 0-9.4 11.9 13.1 13.6 14.5 14.8 15.3 16.6 17.5 17.1 17.3 19.6 9.5-13.5 11.9 12.6 13.1 13.5 14.3 14.8 15.1 16.7 16.7 17.1 20.0 13.5-18.0 12.6 13.0 13.9 14.1 14.3 15.4 15.5 16.5 16.9 17.4 20.0 >18 13.8 14.2 14.8 15.6 15.9 16.4 17.4 16.9 19.1 18.9 21.8

Deleted: 2

Deleted: -299

16

Table II cont’d. Kotelchuck Prenatal Care Index

Adequate plus 13.6 14.3 14.8 15.6 15.9 17.0 16.3 18.8 17.9 18.9 21.6 Adequate 12.5 12.8 13.2 13.8 14.5 14.7 16.2 15.9 17.2 17.5 19.7 Intermediate 12.0 13.2 14.1 15.1 14.3 15.2 16.8 17.6 17.1 16.4 20.2 Inadequate 11.5 12.3 13.5 13.2 14.3 15.9 16.7 15.5 18.5 14.5 19.0 Unknown 11.3 12.6 14.2 13.6 14.2 14.8 17.1 17.0 18.9 18.4 20.1

Smoking during pregnancy Yes 11.9 12.3 12.8 13.2 14.6 15.1 14.8 16.1 15.7 16.2 19.5 No 12.6 13.4 14.0 14.7 15.0 15.8 16.8 17.5 18.2 18.1 20.8 Unknown 14.2 15.9 15.4 17.9 14.3 17.5 22.4 14.9 23.2 24.0 20.3

Previous cesarean delivery No 11.2 11.9 12.5 13.1 13.7 14.4 15.2 16.5 16.7 16.9 19.9 Yes 19.1 19.9 20.1 20.2 20.2 21.2 21.3 19.8 21.7 21.2 22.8

Deleted: 2

17

Table III. Adjusted odds ratios* for labor dystocia (overall, functional, mechanical, and their constituents) associated with every 12 months of increment in interpregnancy interval: Matched birth-hospital discharge data, Michigan, 1994-2002.

Adjusted Odds Ratio 95% Confidence Interval Labor Dystocia

1.0359 1.0331 - 1.0387 Functional Dystocia

1.0469 1.0431 - 1.0507 Delayed delivery 1.0396 1.0319 - 1.0473 Failed induction 1.0708 1.0615 - 1.0801 Uterine inertia or abnormal uterine contractions 1.0487 1.0440 - 1.0534 Prolonged labor 1.0401 1.0256 - 1.0547 Mechanical Dystocia

1.0297 1.0263 - 1.0331 Malposition or malpresentation of fetus

1.0231 1.0188 - 1.0275 Obstructed labor 1.0367 1.0318 - 1.0416 Disproportion 1.0513 1.0423 - 1.0605 * Controlled for infant’s birth weight (<1500, 1500-2499, 2500-3999, 4000+g); mother’s age, race, marital status, education (<9, 9-11, 12, 13-15, 16+ years), total number of previously recognized pregnancies, outcome of the preceding pregnancy (live birth or other terminations), weight gain during pregnancy, prenatal care utilization, smoking during pregnancy, and previous cesarean delivery.

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Uterine inertia / abnormal uterine contractions Prolonged labor

Malposition or malpresentation of fetus Obstructed labor

Disproportion

This piece of the submission is being sent via mail.

Copyright Agreement - already sent with original submission

1




Thu Le, PhD b

Mei Lin, MD, MSc a

Glenn Copeland, MA b

Maurice Barone b







Word count for text: 2628

Original manuscript submitted on 8/23/05

2

ABSTRACT




to 2002 with the hospital discharge data for this study. A physician panel identified the

International Classification of Diseases (9th revision, clinical modifications, ICD-9-CM) codes

indicating labor dystocia, and classified the codes into functional and mechanical dystocia. We

estimated the prevalence of labor dystocia, and used logistic regression to evaluate labor dystocia

in relation to interpregnancy interval, controlling for other reproductive risk factors.

RESULTS. Labor dystocia was experienced in 20.8% of the births (11.1% functional, and

12.5% mechanical). Both functional and mechanical dystocia were more prevalent in first births

than in higher-order births; mechanical dystocia was more prevalent in multiple births than in

singleton births. In singleton births to multiparous mothers, labor dystocia was linearly related

with the interpregnancy interval: For every year of delayed conception, the risk for overall,

functional, and mechanical dystocia increased by 3.6% (95% confidence interval [CI], 3.3%-

3.9%), 4.7% (95% CI, 4.3%-5.1%), 3.0% (95% CI, 2.6%-3.3%), respectively, controlling for

other reproductive risk factors. The risk increase for the individual constituents of labor dystocia

associated with every year of delayed conception varied from 2.3% (for malposition or

malpresentation) to 7.1% (for failed induction of labor).



interval.

Key words: birth intervals, dystocia; difficult childbirth; difficult labor

3
















services.7 However, overall, the knowledge about the prevalence of labor dystocia, its causes and

risk factors is extremely scant. In this study we sought to identify labor dystocia using the

International Classification of Diseases, 9th revision, clinical modification (ICD-9-CM) codes.8 We

used the livebirth certificate data for a large cohort of births linked to the delivery hospital





4



















Michigan Inpatient Database (Table 1). These conditions were classified into functional dystocia

(including delayed delivery, failed induction, uterine inertia and abnormal uterine contractions,

prolonged labor) and mechanical dystocia (including malposition and malpresentation of the fetus,

obstructed labor, and disproportion). A woman was classified as having labor dystocia if any of

these conditions were among the discharge diagnoses.




5





months.

We evaluated the following risk factors as potential confounding: Infant’s birth weight

(<1500, 1500-2499, 2500-3999, and >4000g); mother’s age (categorized into five-year age

groups in the stratified analyses and used as a continuous variable in logistic regression), race,






infant’s birth certificate). Also, in the sensitivity analysis, the results changed little when we used

the infant’s gestational age (<32, 32-36, 37-41, and >42 completed weeks) instead of birth

weight, and the total number of previous births instead of number of pregnancies (not shown).

We performed stratified analyses of the relationship between labor dystocia (overall,

functional, mechanical, and their constituents) and interpregnancy interval according to the levels

of the above-mentioned confounding factors, and used logistic regression15 to control for the

confounding factors simultaneously. When fitting the logistic regression model on functional

dystocia, we excluded records where mechanical dystocia was indicated; and vise versa. Because

the data for infants born to the same biological mother may be correlated, and this correlation

could result in biased variance estimates and incorrect statistical inferences if not appropriately

accounted for,16 we used the generalized estimating equation (GEE) approach in the logistic

regression analyses with the exchangeable correlation structure.17 Also, we included


6









Results

Between January 1, 1994 and December 31, 2002, 1,210,757 live infants were delivered to

Michigan resident women. Hospital discharge records were identified in the Michigan Inpatient

Database for 1,181,847 (97.6%) of those births. Of the births with matched hospital discharge

data, 16.0% had ICD-9-CM codes indicating labor dystocia; 11.1% had functional dystocia, and

12.5% had mechanical dystocia. (Of note, the percentages of the two types of labor dystocia did

not add up to the percentage of overall labor dystocia because some women had both types of

labor dystocia.) Labor dystocia was more prevalent among first births (29.0% overall, 17.6%

functional, and 16.5% mechanical) than among higher-order births (15.4% overall, 6.9%

functional, and 9.9% mechanical). Also, multiple gestations were twice as likely to experience

overall labor dystocia (42.0% vs. 20.1%) and three times as likely to experience mechanical

dystocia (35.3% vs. 11.8%), but were slightly less likely to experience functional dystocia (9.8%

vs. 11.2%), compared with singleton births.





7










those risk factors where the data supported the stratified analyses (Table 2). Specifically, to


with both interpregnancy interval and labor dystocia, we restricted the analysis to only 30-year-

old mothers and found the same relationship (not shown).


regression, the adjusted odds ratios (aORs) associated with every 12 months of increment in the



mechanical dystocia (Table 3). These data indicate that for every year of delayed conception

following a live birth, the risk for overall, functional, and mechanical dystocia increases by

approximately 3.6%, 4.7%, and 3.0%, respectively. The increase in the risk would become

substantial if the conception is delayed for a prolonged period of time. For example, a five-year

delay in conception would result in a 19% increase in overall dystocia, 26% increase in functional

dystocia, and 16% increase in mechanical dystocia, whereas a 10-year delay in conception would

result in risk increases of 42%, 58%, and 34%, respectively.

Comment

8

Labor dystocia, the opposite of labor eutocia, or normal labor, can be caused by ineffective



with the passage of the fetus through the birth canal. Every year, a large number of women have

labor dystocia during labor and delivery, especially in developing countries, which can have dire

consequences.1 Yet there is insufficient information about the magnitude of the problem and its

related risk factors.




mechanical. We estimated that labor dystocia was experienced by approximately a fifth of all live


more prevalent among first births than among higher-order births; mechanical dystocia, but not

functional dystocia, were more common among multiple births than among singleton births.





reproductive risk factors. To our knowledge, this study is the first attempt to measure labor

dystocia in the general population, and to evaluate the relationship between labor dystocia and

interpregnancy interval.





a long interpregnancy interval has been associated with preeclampsia and eclampsia.9, 20, 21 In

9







theorizes that pregnancy may physiologically prepare and optimize the growth-supporting




consistent with the results of the current study: As the interpregnancy interval increases, the



labor dystocia.





Third, we linked the infants to their biological mothers, allowing us to perform the GEE analysis

to accounting for correlation among biological siblings.




may be subject to error. To evaluate the accuracy of the preceding infant’s birth date reported by



10



26.55; correlation coefficient: 0.999, p<0.0001). Also, gestational age estimated from the last

menstrual period may be subject to various errors.24 However, since the errors in estimated

gestational age (mostly less than a few weeks) are likely to be small relative to the length of the

interpregnancy interval, we expect these errors to have minimal impact on the results of this

study. Second, no hospital discharge data were found for approximately 2.4% of births, many of

which are likely to be home births. These births are probably at lower risk for labor dystocia.

However, since this percentage is small, we expect the resultant bias, if any, to be minimal. Third,

this study used a linked birth and hospital discharge database, which have been shown to provide

accurate estimates for certain obstetric conditions.25 However, whether labor dystocia has been

accurately identified in this study is unknown, although the use of multiple ICD-9-CM codes to

identify labor dystocia may help to reduce the underestimation. Fourth, due to data limitations we

were unable to assess maternal morbidities and other obstetrical variables that may be associated

with both interpregnancy interval and labor dystocia. Fifth, this study is based on the U.S. data,

which may not be generalizable to developing countries, where labor dystocia has the most

serious consequences. For these reasons this study needs to be interpreted with caution, and

replicated in future studies in other settings, especially developing countries.









11

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3. Towner D, Castro MA, Eby-Wilkens E, Gilbert WM. Effect of mode of delivery in nulliparous women on neonatal intracranial injury. N Engl J Med. Dec 2 1999;341(23):1709-1714.

4. Johnson JH, Figueroa R, Garry D, Elimian A, Maulik D. Immediate maternal and neonatal effects of forceps and vacuum-assisted deliveries. Obstet Gynecol. Mar 2004;103(3):513-518.

5. Koroukian SM. Relative risk of postpartum complications in the Ohio Medicaid population: vaginal versus cesarean delivery. Med Care Res Rev. Jun 2004;61(2):203-224.

6. Wen SW, Liu S, Kramer MS, et al. Comparison of maternal and infant outcomes between vacuum extraction and forceps deliveries. Am J Epidemiol. Jan 15 2001;153(2):103-107.

7. Konje JC, Ladipo OA. Nutrition and obstructed labor. Am J Clin Nutr. Jul 2000;72(1 Suppl):291S-297S.


9. Skjaerven R, Wilcox AJ, Lie RT. The interval between pregnancies and the risk of preeclampsia. N Engl J Med. Jan 3 2002;346(1):33-38.

10. Zhu BP, Haines KM, Le T, McGrath-Miller K, Boulton ML. Effect of the interval between pregnancies on perinatal outcomes among white and black women. Am J Obstet Gynecol. Dec 2001;185(6):1403-1410.

11. Zhu BP, Le T. Effect of interpregnancy interval on infant low birth weight: a retrospective cohort study using the Michigan Maternally Linked Birth Database. Matern Child Health J. Sep 2003;7(3):169-178.

12. Zhu BP, Rolfs RT, Nangle BE, Horan JM. Effect of the interval between pregnancies on perinatal outcomes. N Engl J Med. Feb 25 1999;340(8):589-594.

13. Ventura SJ, Martin JA, Curtin SC, Mathews TJ. Report of final natality statistics, 1995. Mon Vital Stat Rep. 1997;45 (Suppl).

14. Kotelchuck M. The Adequacy of Prenatal Care Utilization Index: its US distribution and association with low birthweight. Am J Public Health. Sep 1994;84(9):1486-1489.

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16. Watier L, Richardson S, Hemon D. Accounting for pregnancy dependence in epidemiologic studies of reproductive outcomes. Epidemiology. Nov 1997;8(6):629-636.

17. Zeger SL, Liang KY, Albert PS. Models for longitudinal data: a generalized estimating equation approach. Biometrics. Dec 1988;44(4):1049-1060.

18. Shipp TD, Zelop CM, Repke JT, Cohen A, Lieberman E. Interdelivery interval and risk of symptomatic uterine rupture. Obstet Gynecol. Feb 2001;97(2):175-177.

19. Bujold E, Mehta SH, Bujold C, Gauthier RJ. Interdelivery interval and uterine rupture. Am J Obstet Gynecol. Nov 2002;187(5):1199-1202.

20. Conde-Agudelo A, Belizan JM, Lammers C. Maternal-perinatal morbidity and mortality associated with adolescent pregnancy in Latin America: Cross-sectional study. Am J Obstet Gynecol. Feb 2005;192(2):342-349.

21. Mostello D, Catlin TK, Roman L, Holcomb WL, Jr., Leet T. Preeclampsia in the parous woman: who is at risk? Am J Obstet Gynecol. Aug 2002;187(2):425-429.

22. Miller JE. Birth intervals and perinatal health: an investigation of three hypotheses. Fam Plann Perspect. Mar-Apr 1991;23(2):62-70.

23. Winkvist A, Rasmussen KM, Habicht JP. A new definition of maternal depletion syndrome. Am J Public Health. May 1992;82(5):691-694.

24. Alexander GR, Tompkins ME, Petersen DJ, Hulsey TC, Mor J. Discordance between LMP-based and clinically estimated gestational age: implications for research, programs, and policy. Public Health Rep. Jul-Aug 1995;110(4):395-402.

25. Parrish KM, Holt VL, Connell FA, Williams B, LoGerfo JP. Variations in the accuracy of obstetric procedures and diagnoses on birth records in Washington State, 1989. Am J Epidemiol. Jul 15 1993;138(2):119-127.

13

Table 1. Diagnosis and Procedure Codes Indicating Labor Dystocia, Based on the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) Functional Dystocia Delayed delivery 658.2 Delayed del after spontaneous/unspec rupture of membranes 658.3 Delayed del after artificial rupture of membranes Failed induction 659.0 Failed mechanical induction (surgical/other instrument methods) 659.1 Failed medical/unspecified induction (e.g. oxytocin) 660.6 Failed trial of labor, unspecified 660.7 Failed forceps or vacuum extractor, unspecified Uterine inertia and abnormal uterine contractions 661.0 Primary uterine inertia 661.1 Secondary uterine inertia 661.2 Other and unspecified uterine inertia 661.4 Hypertonic, incoordinate, or prolonged uterine contractions 763.7 Abnormal uterine contractions Prolonged labor 662.0 Prolonged first stage 662.1 Prolonged labor, unspecified 662.2 Prolonged second stage Mechanical Dystocia Malposition and malpresentation of fetus 652.0 Unstable lie 652.1 Breech or other malpresentation successfully converted to cephalic presentation 652.2 Breech presentation without mention of version 652.3 Transverse or oblique presentation 652.4 Face or brow presentation 652.5 High head at term 652.7 Prolapsed arm 652.8 Other specified malposition or malpresentation 652.9 Unspecified malposition or malpresentation Obstructed labor 660.0 Obstruction caused by malposition of fetus at onset of labor 660.1 Obstruction by bony pelvis 660.2 Obstruction by abnormal pelvic soft tissues 660.3 Deep transverse arrest and persistent occipitoposterior position 660.4 Shoulder (girdle) dystocia 660.8 Other causes of obstructed labor 660.9 Unspecified obstructed labor Disproportion 653.0 Major abnormality of bony pelvis, not further specified 653.1 Generally contracted pelvis 653.2 Inlet contraction of pelvis 653.3 Outlet contraction of pelvis 653.4 Fetopelvic disproportion 653.5 Unusually large fetus causing disproportion 653.6 Hydrocephalic fetus causing disproportion 653.7 Other fetal abnormality causing disproportion


14

Table 2. Percent of labor dystocia by interpregnancy interval according to levels of selected reproductive risk factors: Singleton live births to multiparous mothers, Michigan, 1994-2002 Interpregnancy Interval (months) 0-11 12-23 24-35 36-47 48-59 60-71 72-83 84-95 96-107 108-119 120-299Overall 12.5 13.2 13.8 14.4 14.9 15.6 16.4 17.1 17.6 17.7 20.4Infant birthweight (g)

<1500 45.8 40.6 41.1 45.1 43.2 44.1 48.1 43.5 46.5 48.8 47.21500-<2500 18.3 20.5 20.9 21.4 21.0 21.4 22.1 24.2 21.6 22.3 24.22500-<4000 10.6 11.3 12.0 12.5 12.8 13.7 13.9 14.7 15.4 15.5 17.84000+ 20.1 21.1 21.4 22.4 24.0 23.5 27.2 26.8 27.8 26.7 31.5

Mother age (yr) 10-19 11.3 11.7 11.4 14.8 15.5 11.4 18.8 50.0 . . .20-24 11.4 12.4 12.7 13.2 13.8 15.3 15.4 15.4 17.4 17.9 23.125-29 12.3 13.1 13.8 14.2 14.6 15.0 15.6 16.5 17.0 16.6 18.630-34 13.7 13.7 14.3 15.1 15.4 16.1 16.2 17.4 18.1 17.5 19.435-39 15.5 14.8 15.0 15.5 15.7 16.1 18.1 17.7 18.1 18.1 20.740+ 17.2 15.5 17.3 15.7 18.0 19.2 19.0 19.1 15.8 20.4 22.5

Mother's race White 12.9 13.4 14.0 14.5 15.0 15.6 16.3 17.1 17.6 17.8 20.1Black 11.2 12.3 13.1 14.0 14.6 15.6 16.5 17.5 17.9 16.7 20.9Native American 12.7 13.8 16.3 13.9 19.8 23.7 18.2 16.8 7.6 18.4 27.5Asian/Pacific Islander 11.5 12.4 13.7 15.6 14.4 16.0 16.1 12.3 18.2 19.9 22.4Other/Unknown 14.5 14.2 16.0 20.8 17.4 15.2 20.5 21.1 14.9 38.9 22.4

Mother's marital status Married 13.1 13.6 14.2 14.8 15.2 15.7 16.6 17.4 18.0 18.5 20.5Other 11.5 12.1 12.6 13.6 14.4 15.5 16.0 16.7 17.0 16.1 20.3

Mother's education (yrs) 0-8 11.7 12.5 12.8 13.8 13.0 17.0 13.9 18.0 17.5 21.9 22.19-11 11.3 12.1 12.4 13.5 14.1 15.1 14.3 15.2 16.3 16.2 20.312 12.1 13.1 13.6 14.5 14.7 15.2 16.3 17.0 17.9 17.4 20.313-15 13.0 13.8 14.4 14.5 15.5 16.0 16.8 17.8 17.4 17.3 20.316+ 14.1 13.5 14.4 14.8 15.1 16.1 17.3 17.3 18.5 19.1 20.9

Number of previous pregnancies 1 13.9 14.0 14.9 15.7 16.0 16.6 16.8 18.4 19.7 18.3 21.02 11.8 12.9 13.0 13.8 14.2 14.8 16.6 16.6 16.0 18.0 19.93 11.3 12.4 12.9 13.7 13.9 15.2 16.0 15.3 16.7 15.0 20.14 11.3 12.1 13.7 12.8 14.1 16.4 14.4 17.2 17.0 18.5 20.25+ 12.1 12.7 13.0 14.6 16.0 15.4 16.9 18.5 18.4 19.2 20.7

Outcome of last pregnancy Live birth 12.5 13.2 13.8 14.4 14.8 15.5 16.2 17.0 17.7 17.4 20.2Other terminations 13.1 13.8 14.4 14.6 15.3 16.2 16.8 17.6 17.5 18.8 21.1

Weight gain during pregnancy (kg) 0-9.4 11.9 13.1 13.6 14.5 14.8 15.3 16.6 17.5 17.1 17.3 19.69.5-13.5 11.9 12.6 13.1 13.5 14.3 14.8 15.1 16.7 16.7 17.1 20.013.5-18.0 12.6 13.0 13.9 14.1 14.3 15.4 15.5 16.5 16.9 17.4 20.0>18 13.8 14.2 14.8 15.6 15.9 16.4 17.4 16.9 19.1 18.9 21.8

15

Table 2 cont’d. Kotelchuck Prenatal Care Index

Adequate plus 13.6 14.3 14.8 15.6 15.9 17.0 16.3 18.8 17.9 18.9 21.6Adequate 12.5 12.8 13.2 13.8 14.5 14.7 16.2 15.9 17.2 17.5 19.7Intermed 12.0 13.2 14.1 15.1 14.3 15.2 16.8 17.6 17.1 16.4 20.2Inadequate 11.5 12.3 13.5 13.2 14.3 15.9 16.7 15.5 18.5 14.5 19.0Unknown 11.3 12.6 14.2 13.6 14.2 14.8 17.1 17.0 18.9 18.4 20.1

Smoking during pregnancy Yes 11.9 12.3 12.8 13.2 14.6 15.1 14.8 16.1 15.7 16.2 19.5No 12.6 13.4 14.0 14.7 15.0 15.8 16.8 17.5 18.2 18.1 20.8Unknown 14.2 15.9 15.4 17.9 14.3 17.5 22.4 14.9 23.2 24.0 20.3

Previous cesarean delivery No 11.2 11.9 12.5 13.1 13.7 14.4 15.2 16.5 16.7 16.9 19.9Yes 19.1 19.9 20.1 20.2 20.2 21.2 21.3 19.8 21.7 21.2 22.8

16

Table 3. Adjusted odds ratios* for labor dystocia (overall, functional, mechanical, and their constituents) associated with every 12 months of increment in interpregnancy interval: Matched birth-hospital discharge data, Michigan, 1994-2002.

Adjusted Odds Ratio 95% Confidence Interval Labor Dystocia

1.0359 1.0331 - 1.0387 Functional Dystocia

1.0469 1.0431 - 1.0507 Delayed delivery 1.0396 1.0319 - 1.0473 Failed induction 1.0708 1.0615 - 1.0801 Uterine inertia and abnormal uterine contractions 1.0487 1.0440 - 1.0534 Prolonged labor 1.0401 1.0256 - 1.0547 Mechanical Dystocia

1.0297 1.0263 - 1.0331 Malposition and malpresentation of fetus

1.0231 1.0188 - 1.0275 Obstructed labor 1.0367 1.0318 - 1.0416 Disproportion 1.0513 1.0423 - 1.0605 * Controlled for infant’s birth weight (<1500, 1500-2499, 2500-3999, 4000+g); mother’s age, race, marital status, education (<9, 9-11, 12, 13-15, 16+ years), total number of previously recognized pregnancies, outcome of the preceding pregnancy (live birth or other terminations), weight gain during pregnancy, prenatal care utilization, smoking during pregnancy, and previous cesarean delivery.

17


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20.0

25.0

0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102 108 114 120 126 132 138 144 150


%


18


0.01.02.03.04.05.06.07.08.09.0

10.0


%


Uterine inertia / abnormal uterine contractions Prolonged labor

Malposition or malpresentation of fetus Obstructed labor

Disproportion

Date post:	29-Nov-2023
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Title: Effect of the Interval Between Pregnancies on Labor Dystocia

Documents