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Obstetrical Ultrasound
and Fetal Abnormalities
Chapter 17
Amelia McLennan, MD
❍ How is ultrasound produced?
It is the vibrations of crystals in response to electrical current generate sound waves. This is known as the
piezoelectric effect.
❍ True or False: The frequencies of the transvaginal transducers are typically higher than the transabdominal.
True. The transabdominal probes are typically 3 to 7 MHz and the transvaginal probes 5 to 9 MHz. With higher
frequencies, there is greater resolution but decreased penetration.
❍ Is ultrasound safe?
With the widespread use, no biological effects have been confirmed on the fetus with the frequencies used in
obstetrical ultrasound. The level of ultrasound intensity that is defined as safe is <100 mW/cm^2. It has been shown
to cause thermal effects and mechanical changes via cavitation in animal studies.
❍ What serum analytes are used in conjunction with ultrasound to screen for aneuploidy?
In the first trimester serum pregnancy-associated plasma protein A (PAPP-A) and free beta human chorionic
gonadotropin (hCG) levels and in the second trimester unconjugated estriol, beta hCG and inhibin A are also
measured. In Down syndrome, the serum beta hCG (free and total) and inhibin A levels are increased and the
levels of PAPP-A, uE3, and AFP are decreased.
❍ Increased nuchal translucency is associated with which fetal abnormalities?
Trisomy 13, 18, and 21.
❍ What is the most accurate measurement to estimate gestational age in first trimester?
Crown-rump length is better than the gestational sac and yolk sac measurements.
❍ What is the most accurate measurement to estimate gestational age in the second trimester?
Head circumference.
❍ At what crown-rump length should embryonic cardiac activity be observed?
5 mm or greater.