Principles and Applications of Ultrasound to Obstetrics

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Principles and Applications of Ultrasound to
Obstetrics

• Honor 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 8 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 lt
10 wks 3-5 days gt 10 wks less
accurate variable 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 diameter Head
circumference Femur length Abdominal
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
• gt 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
penetration Directly proportional to
resolution Probes Transabdominal 3.5, 5, 7
mHz Transvaginal 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 fetal movements
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
measureslt 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 (gt 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

D Kell Duffy MNS Kidd
Lutheran
Queenan et al. Obstet Gynecol 1969 34
767-70 Geifman-Holtzman et al. Obstet Gynecol
1997 89 272-5
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ACOG recommends antenatal RHIG
ACOG recommends antenatal 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
101297-300
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RED CELL ALLOIMMUNIZATIONNew 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 ALLOIMMUNIZATIONNew 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 ?OD450 testing

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RED CELL ALLOIMMUNIZATIONPrevious RhD
Sensitization

• History of previous IUFD, intrauterine
  transfusions or neonatal exchange
  transfusions
• Maternal titers not helpful