Principles and Applications of Principles and Applications of Ultrasound to ObstetricsUltrasound to Obstetrics

Honor M. WolfeHonor M. Wolfe

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What is the accuracy of ultrasound in the assessment

of gestational age?

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GA Assessment

Accuracy ∞ 1/Gestational Age

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Gestational age accuracy

1st trimester + 1 week

2nd trimester + 2 weeks

3rd trimester + 3 weeks

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First Trimester: CRL

5-12 weeks gestation

< 10 wks + 3-5 days> 10 wks

less accuratevariable position/flexion

5-7 days

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2nd and 3rd trimester

Accuracy of GA estimates increases as more variables are measured.

- Composite estimate of:Biparietal diameterHead circumferenceFemur lengthAbdominal circumference

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Does maternal BMI impact ultrasound and if so how and

why?

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Physics

• High frequency sound waves– > 20,000 cycles/second

• Frequency– Number of waves per unit time– Expressed as hertz (Hz)

• Diagnostic ultrasound– 2-10 million Hz (2-10 MHz)

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Physics

Frequency Inversely proportional to penetrationDirectly proportional to resolution

Probes Transabdominal 3.5, 5, 7 mHzTransvaginal 8-9 mHz

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Sound waves

- Transducer both sends and receives

- Reflected by emitting transducer

- Image displayed as:

1. Brightness - intensity of echo 2. Time lag - distance

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Ultrasound and BMI

• Heavier patients – Need more penetration (lower mHz)– Get less resolution (lower mHz)

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What are the types of US – who gets what type of scan?

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Basic Ultrasound

• Examination– Fetal number/presentation/”life”– Placental location– Assessment of AFV– Assessment of gestational age– Survey for “gross” malformations– Evaluation for maternal pelvic masses

• Metric examination• Screening

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Limited Ultrasound

– Assessment of AFV, BPP– Guidance for

• Amniocentesis• External cephalic version

– Confirmation of fetal death– Placental localization (hemorrhage)– Fetal presentation

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Comprehensive Ultrasound

• Indications– Suspicion of anomalous fetus

• History• Clinical evaluation• Previous ultrasound

• Detailed assessment of fetal anatomy– Color/power doppler– Arterial/venous doppler

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What type of anomalies is this patient at risk for and how good is

ultrasound at finding them?

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How good is ultrasound at finding anomalies?

It depends on:

The anomaly

Minor anomalies, heart anomalies hardest

When we look

When apparent, 20 –24 wks optimal for most

Who we are looking at

Thinner, normal amniotic fluid volume

And…….

Who is looking.

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Detection

Directly proportional to severity of anomaly

- 89% lethal anomalies

- 77% requiring NICU admission

- 30% minor anomalies

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Lowest rates

Cardiovascular defects

Cleft up / palate

Microcephalus

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Types of Ultrasound – what might be missed?

• Basic (76805)• Measurements, AFI,

placenta• Head• Heart (not color)• Abdomen

• Comprehensive (76811)

• Face, profile• Extremities• Heart

– Color doppler– Extremities

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What about antenatal testing?

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Table 43-1. COMPONENTS AND THEIR SCORES OF THE BIOPHYSICAL PROFILE

Variable Score 2 Score 0

Fetal breathing The presence of at least 30 sec of sustained fetalmovements breathing movements in 30 min of observation

Less than 30 sec of fetal breathing movements in 30 min

Fetal movements Three or more gross body movements in 30 min of Two or less gross body movement observation: simultaneous limb and trunk movements in 30 min of observation

Fetal tone At least one episode of motion of a limb from position Fetus in position of semi- or of flexion to extension and rapid return to flexionS full-limb extension with no return

or slow return to flexion with movement; absence of fetal movement counted as absent tone.

Fetal reactivity Two or more fetal heart rate accelerations of least No acceleration or less than 15 beats/min and lasting at least 15 sec and associated two accelerations of fetal with fetal movement in 20 min heart rate in 20 min of observation

Qualitative amnionic Pocket of amnionic fluid that measures at least 1 cm Largest pocket of amnionic fluid fluid volume in two perpendicular planes measures< 1 cm in two perpendicular planes

From Manning and colleagues (1985), with permission.

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How well do we estimate fetal weight?

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Estimated Fetal Weight

• Various formulas– All involve the abdominal circumference– Also Femur length, head circumference and/or

BPD

• Less Accurate in bigger babies (> 4000 grams)

• Accuracy + 10 – 15%– Term harder to get measurements– Fetal position AFI

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• Figure 1 (No legend p 524 OB Gyn 1999: 93: 523-6) put in author and year

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RED CELL ALLOIMMUNIZATIONFrequency of Irregular

Antibodies

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5

10

15

20

25

30

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40

45

50

1967

1996%

D Kell Duffy MNS Kidd Lutheran

Queenan et al. Obstet Gynecol 1969; 34: 767-70Geifman-Holtzman et al. Obstet Gynecol 1997; 89: 272-5

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ACOG recommends antenatal RHIG

ACOG recommendsantenatal RHIG

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RED CELL ALLOIMMUNIZATIONRhesus Prophylaxis

• 66% of Rhesus cases – antepartum sensitization

• 13% of cases – inadvertent omission of RhIG

Hughes et al. Brit J Obstet Gynaecol 1994; 101:297-300

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RED CELL ALLOIMMUNIZATION

New Onset RhD Sensitization• Follow maternal titers every 2 - 4

weeks until critical value reached (32 at UNC)

• Determine paternal genotype for involved antigen

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RED CELL ALLOIMMUNIZATION

New Onset RhD Sensitization• Paternal genotype = heterozygous

(55%); do amniocentesis for fetal blood typing

• Paternal genotype = homozygous (45%) or affected fetus by amniocentesis DNA testing; begin serial amniocenteses for ΔOD450 testing

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RED CELL ALLOIMMUNIZATION

Previous RhD Sensitization• History of previous IUFD, intrauterine transfusions or neonatal exchange transfusions

• Maternal titers not helpful