The role of ultrasound in screening for fetal Down syndrome.

Antenatal Down syndrome screening begins with maternal age-specific risks. It also may incorporate the maternal serum screen and antenatal ultrasound. These complementary tests improve the efficacy of maternal age in Down syndrome screening. Ultrasound enhances antenatal Down syndrome screening by 1) precise dating of a pregnancy for the interpretation of serum screen results; 2) detecting major structural anomalies that occur in approximately 20% of all second-trimester Down syndrome fetuses; 3) detecting minor markers, such as nuchal fold thickness, pyelectasis, echogenic bowel, short long bones, sandal gap, and hypoplasia of the mid-phalanx of the fifth digit, which are more commonly seen in second-trimester fetuses with Down syndrome; and 4) acting in combination with serum screening in the first trimester using fetal nuchal translucency as the marker. Ultrasound enhances the efficacy of either age or age and the serum screen for the antenatal detection of Down syndrome.