180 Obstetrics and Gynecology Board Review •••
❍ If only one ultrasound can be done, what is the optimal gestational age at which it should be done?
18 to 20 weeks.
❍ What parameters are commonly used to assess gestational age in the second and third trimester?
Biparietal diameter, head circumference, abdominal circumference, and femur length.
❍ How is the biparietal diameter measured?
From the outer edge of the proximal skull to the inner edge of the distal skull at the level of the thalami and cavum
septi pellucidi. The cerebellar hemispheres should not be visible. The head circumference is also measured at this
level.
❍ At what level is the abdominal circumference measured?
The image should be at the level of the junction of the umbilical vein and portal sinus with the fetal stomach
visible.
❍ Why does femur appear to be bowed in normal fetus?
It is the inability to see the full thickness of the femoral diaphysis that gives the impression that the femur farther
away from the transducer is bowed. The femoral length should only include the diaphysis and metaphysis.
❍ After what gestational age can femur length be accurately measured?
14 weeks.
❍ What is asymmetric intrauterine growth restriction (IUGR)?
Estimated fetal weight less than the 10th percentile with decreased ratio of abdominal circumference to head
circumference.
❍ What is the margin of error of estimated fetal weight by ultrasonography in the second and third trimesters?
15% to 20%.
❍ Which Doppler study is used to follow fetuses with IUGR?
Umbilical artery velocimetry. It has also been shown that the increase in flow resistance in the umbilical artery is
correlated with decreased flow resistance in the middle cerebral artery (MCA) and this has been attributed to the
brain sparing response of the IUGR fetus.
❍ Which imaging modality measures fetal anemia?
MCA Doppler. With anemia there is increased fetal cardiac output, which is attributed to decreased blood viscosity
and decreased peripheral vascular resistance. These allow for delivery of larger volume of less oxygenated blood.
Hence, the blood flow velocity in the MCA is increased.
❍ What Doppler measurement must be followed when indomethacin is given for preterm labor?
Ductus arteriosus. Indomethacin has been shown to cause premature constriction of the ductus arteriosus. This
is seen by increase in peak systolic velocity (PSV) and decrease in pulsatility index (PI). The effect is considered
reversible and abnormal Doppler should lead to discontinuation of the indomethacin.