Surveillance of twin pregnancy with umbilical

arterial velocimetry

George Farmakides, M.D., Harold Schulman, M.D., Luis R. Saldana, M.D., Luis A. Bracero, M.D., Adiel Fleischer, M.D., and Burton Rochelson, M.D.

Mineola, Stony Brook, and Bronx, New York, and Cincinnati, Ohio

A continuous-wave Doppler system and a spectrum analyzer provide the tools for the measurement of

systolic-diastolic velocity wave ratios in the umbilical arteries. In twins, each umbilical cord can be studied

without imaging the fetuses. Results were tabulated by taking the average difference in the ratios between each fetus and plotting these values against the neonatal weight differences. In 43 twin pregnancies, the data revealed that ratio differences between fetuses that averaged 0.4 or more was predictive of a weight

difference of >349 gm. The sensitivity was 73% and specificity 82%. Two cases of twin transfusion syndrome were recognized by the simultaneous presence of high- and low-resistance values. (AM J 0BSTET GYNECOL 1985;153:789-92.)

Key words: Doppler, umbilical artery, twin, systolic/diastolic ratios

The appropriate surveillance of the twin pregnancy represents an unresolved and challenging chapter in obstetrics. The virtues of ultrasound imaging and fetal heart rate monitoring have been expressed, but un­fortunately these methodologies have limitations. 1· 1

These biophysical techniques are more difficult to in­terpret in twin pregnancies as compared with preg­nancies with a single fetus because of the overlapping bodies.

Although the twin pregnancy is susceptible to all the difficulties of the singleton gestation, the unique prob­lem for the twin is that of placentation.5 Hence, a clinical tool that could measure the fetal-placental circulation would be useful in identifying the circumstance in which a growth disorder or fistulous communication were present. Such technology is now available." Um­bilical arterial velocimetry carried out by continuous­wave Doppler ultrasound can define a normal umbilical circulation versus one in which there is increased or decreased vascular resistance. It has been demonstrated that umbilical blood How is decreased and arterial re­sistance is increased in most cases of fetal growth re­tardation. 7·

8 This report summarizes our first experi­ences when twin pregnancies were subjected to serial umbilical velocimetry. The technique appears to fulfill its promise of providing important information that

From the Department of Obstetrics and Gynecology, Winthrop Uni­versity Hospital (formerly Nassau Hospital), Mineola, the Stale University of New York al Stony Brook, the Department of Obstetncs and Gynecology, Albert Einstein College of Medicine, Bronx, and the Department of Obstetrics and Gynecology, Bethesda Hospital, Cincinnati.

Received for publication May 16, 1985; revised September 5, 1985; accepted September 18, 1985.

Reprint requests: George Famuikides, M.D., Winthrop University Hospital, 259 First St., Mineola, NY 11501.

should lead to improved management of the twin ges­tation.

Material and methods

The umbilical arterial velocity waveform was studied in 43 pairs of twins. The study was carried out with an Angioscan spectrum analyzer. The specifications and techniques of the method were described in previous publications."·'

The systolic/diastolic ratio was measured for each pair of twins. The Doppler transducer was placed on the lateral half of the uterus to identify one of the twins. After a signal was detected it was stored on a tape recorder. A minimum of 12 equal waveforms were ob­tained before an average systolic/diastolic ratio was cal­culated. The second cord was then searched for on the opposite side of the uterus. Efforts were made to ensure that the Doppler probe was pointing in a different di­rection. In most circumstances it is relatively easy to recognize two separate cords by the differences in sound and rate. A ratio was calculated for the second cord and then subtracted from the systolic/diastolic ra­tio obtained for the first cord. This was termed the delta systolic/diastolic ratio. The delta measurement circum­vented the problem of the twins moving into different positions during subsequent measurements, a problem not solved by real-time ultrasound. If there were dif­ficulties locating the cords, real-time ultrasound was used. Real-time ultrasound was also used for estimation of gestational age and for the diagnosis of growth re­tardation. In no circumstance were the umbilical ve­locity data used for clinical decision making.

When the twins were delivered, the placentas were weighed. Milk was injected into monochorionic placen­tas to detect vascular communications." The neonates

789

790 Farmakides et al. December I, 1985 Am J Obstet Gynecol

A' both neonates SGA •' one neonate SGA

1.4 Positive test fl. SID~ 0 4

0 Positive results fl. weight~ 350gm

12 (5) FP

" (11) TP

"' 1.0

z 0

~ 0.8 0 • .... 0 0 0

• 0 ..... 0.6 "' <I 0 • Ix • 0 0.4 0 (23) TN Oe 0

0 0 0 0 0 0 00 0 (4) FN

0 0 0 0.2 0 0 0 0 0

0 0 0

50 100 150 200 250 300 350 400 450 500 550 600 650

fl WEIGHT OF TWIN NEONATES (gms)

Fig. I. Data display of differences of umbilical arterial ratios (average of all values) and neonatal birth weight. Note that the distribution of true negative results has fewer weight differences than the true positive results. Derived definitions are in text. Open circles identify both babies as appropriate for gestational age.

Table I. Results and predictive values

Positive lest Negative lest Total

No. with disease II 4 15 No. without disease 5 23 28 Total 16 27 43

Sensitivity: 11115 = 73%. Specificity: 23/28 = 82%. Defi­nitions: Disease positive when change in weight ;:;. 350 gm.

Positive test if mean change in systolic/diastolic ratio ;:;. 0.4. Predictive value of positive test = 69%.

were weighed, a routine hematocrit was determined, and body length and head circumference were mea­sured. The gestational age was assigned according to Dubowitz criteria.

Statistical measurements carried out were analysis of variance and predictive value and efficiency of medical diagnoses. In our previous studies it was shown that the measurement of a systolic/diastolic ratio has a max­imum error of I6%.

Results

Thirty-five sets of twins were delivered at 36 weeks. The smallest twins were delivered at 33 + weeks and one baby had severe respiratory distress syndrome. There were no neonatal deaths and no major congen­ital abnormalities.

Two cases of twin-twin transfusion were diagnosed. There were a total of 235 examinations. In I I sets of twins there was only a single study. There was one false positive result in this group.

Fig. I presents results in 43 pairs of twins in which the average difference of the systolic/diastolic ratio be­tween 30 and 36 weeks was compared with the weight difference between the neonates. Cutoff points were

established for the ~ systolic/diastolic ratio and weights by the mean + l.95 (SE). With these figures discord­ance was defined as a weight difference of >349 gm and a ratio difference of :;.0.4 (Fig. 2). There were I I pairs in which the test predicted the presence of dis­cordance correctly and five pairs in which it did not.

Seven of the placentas were monochorionic diam­niotic and 36 were dichorionic diamniotic. Two of the monochorionic placentas had major vascular fistulas.

We used the criteria of Naeye"' to categorize the small for gestational age twin. There were seven pairs in which one of the twins was small for gestational age (circle) and one pair in which both of the twins were small for gestational age (triangle) (Fig. I).

In Table I it is seen that the sensitivity is 73%, the specificity 82%, and the predictive value of a positive test 68%.

Satisfactory measurements of standard parameters (biparietal diameter, abdominal circumference, femo­ral length) were obtained by ultrasound in only 70% of the cases. When comparisons were made between sin­gleton and twin pregnancy umbilical velocity wave ra­tios, significant differences were noted (Table II). The average ratio for a singleton appropriate for gestational age twin was significantly higher (p < 0.00 I) than the value for a comparable single fetus. In addition, the twin small for gestational age comparisons were sig­nificantly different from those for twin appropriate for gestational age fetuses.

Two cases of twin-twin transfusion were identified. The first case is not included in our data in Fig. I.

Case reports Case 1. A 27-year-old woman, gravida I, para 0, had

a real-time ultrasound examination at 20 weeks for a

Volume 153 Number 7

Twins and umbilical artery velocimetry 791

27 weeks

L Twin SID 3.8

A Twin SID 4.8

Fig. 2. Umbilical arterial velocity waveforms of discordant twins. Frequency amplitude in kilohertz is relative and angle-dependent. Ratio difference of 0.4 forecasts significant neonatal weight dif­ference. Time frame = 1.6 seconds.

Table II. Comparison of umbilical arterial velocity ratios between singleton and twin pregnancies

Systolic/ diastolic ratio (mean ± SD)

Pregnancy Appropriate for gestational age I n I Small for gestational age I n Significance

Singleton 2.50 ± 0.45 129 3.54 ± 0.63 29 p < 0.001 Twin 2.72 ± 0.54* 198 3.28 ± 0.75 37 p < 0.001

*p < 0.001, singleton appropriate for gestational age fetus compared with twin fetus.

large-for-dates pregnancy. Ultrasound showed a twin pregnancy with marked polyhydramnios and discrep­ancy of abdominal diameters. An Angioscan was done and showed systolic/diastolic values of 4.5 and 2.65. Both twins died in utero in the next 2Y2 weeks. The pathology report showed a large recipient twin, with a low ratio, and a small donor twin, with a high ratio. The placenta was monochorionic diamniotic with ve­nous-arterial communication.

Case 2. A 24-year-old woman, para 0, was evaluated with Angioscan studies from 33 weeks of pregnancy. During labor, twin B developed tachycardia. Twin A had a normal heart rate. In twin B the systolic/diastolic ratio increased sharply to a level of 4. l. Placentas were monochorionic diamniotic, with an artery-artery com­munication. Hematocrit was 55% in twin B and 33% in twin A. Nursery course was normal.

Comment The twin pregnancy is uniquely susceptible to prob­

lems related to disorders of placentation. The presence of one or two placentas, monochorionic and dichorionic pregnancies, and the possible effect of implantation sites all contribute to the potential for fetal growth dis­turbances. This study also demonstrates that in the nor­mal, appropriate for gestational age twin fetus, there is an elevated umbilical vascular resistance when com­pared with that in the singleton fetus. This may explain the observed lag in growth seen in the twin fetus after

the twenty-sixth week of pregnancy. 1" 12

Previous reports have highlighted the apparent jeop­ardy created when there is discordant growth between fetuses. 1"

12 We did not verify this in our study, perhaps because of sample size.

Introduction and availability of ultrasound and elec­tronic fetal heart rate monitoring have provided the physician with the tools to aggressively pursue an op­timum outcome for both fetuses. If one accepts the premise that early diagnosis should lead to improved management and outcome, then umbilical arterial ve­locimetry appears to be an important addition to the surveillance of the twin pregnancy. As this study dem­onstrates, the measurement of umbilical arterial veloc­ity waveforms identifies discordant growth and the presence of arteriovenous interplacental fistulas. 14

The limitations of this new modality are those that would be expected. Umbilical arterial systolic/diastolic ratios reflect the presence of normal or abnormal pla­cental vascular resistance, not fetal weight.

Since vascular resistance is inversely related to flow, in most cases diminished flow will be present when ratios are increased. The method will not be able to identify the compensatory mechanisms of the fetus, such as increased arterial pressure or increased venous­arterial uptake. In our previous studies we selected a systolic/diastolic ratio >3 as a positive test to identify the growth-retarded fetus. Given this value in a single­ton pregnancy, intrauterine growth retardation will be present 49% of the time (predictive value of a positive test). In contrast, when a normal value is present the chance of having a growth-retarded fetus is only 5% (predictive value of a negative test).

Several obstacles are present in examinations of the twin pregnancy. First, when doing sequential tests through the last IO weeks of pregnancy, one can never be certain that the same fetus is being examined each time. Real-time ultrasound did not completely resolve

792 Farmakides et al.

this problem. Hence, in this analysis we chose to com­pute a delta value between the two umbilical cord sys­tolic/diastolic ratios and calculate a weight difference. It was found, when the average systolic/diastolic ratio of all determinations in the last 2 months was contrasted against the birth weight differences, that a critical level of 0.4 predicted a weight difference of ~350 gm. If both babies were growth retarded, this guideline did not apply. As with all tests, there were five false positive results and four false negative ones.

Another problem is that the average systolic/diastolic ratio for the appropriate for gestational age twin is higher than that of the comparable singleton fetus, and the small for gestational age twin's ratio is lower than that of the singleton fetus. This suggests that in twins the smaller baby has a symmetrical growth retardation.

Although several reports have given strong support to the value of ultrasound in monitoring discordant fetal growth, in our laboratories, with results from three different institutions, we found that in at least 30% of cases incomplete examinations occur. These in­clude inability to obtain important parameters such as biparietal diameter, abdominal circumference, and femoral length.

Similarly antepartum fetal heart rate monitoring suf­fers from the limitation of the inability to obtain reliable and technically satisfactory recordings because of fetal movements and positional changes.

Umbilical velocimetry appears to add to our surveil­lance informational data base. In the singleton fetus it identities the fetus at risk for growth retardation as early as 20 weeks' gestation and in this study the twin gestation at 30 weeks or beyond. By clearly identifying the fetus with faulty placental circulation, the ultrason­ographer can more effectively look at one fetus for a biophysical profile and the cardiotocographer can focus on the fetal heart rates. We conclude that umbilical

Derember I, I 985 Am .J Ob,tel Crncrnl

velocimetry is an important advance in fetal surveil­lance and should be applied to the management of all twin and triplet pregnancies from the thirtieth week of pregnancy onward. For diagnosis of twin-twin trans­fusion syndrome, earlier surveillance will probably be useful.

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