First-Trimester Screening for Trisomies 21 and 18

In this multicenter study of first-trimester screening for trisomies 21 and 18, a combination of maternal age, maternal levels of free β human chorionic gonadotropin and pregnancy-associated plasma protein A, and ultrasonographic measurement of fetal nuchal translucency detected 78.7 percent of fetuses with trisomy 21, with a false positive rate of 5 percent. Such screening was also highly sensitive and specific for the identification of fetuses with trisomy 18. Combined screening during the first trimester has good sensitivity. Screening for fetal aneuploidy has evolved from a risk assessment based on maternal age to the present approach, which uses age and levels of maternal serum analytes. In the second trimester, levels of alpha-fetoprotein, human chorionic gonadotropin, and unconjugated estriol are used to refine estimates of risk based on maternal age, and this approach will identify approximately 65 percent of fetuses with Down's syndrome (trisomy 21), with a false positive rate of 4.5 to 5.0 percent. 1 Measurement of inhibin A has been added by many laboratories and may increase the rate of detection to 75 percent. 2 Serum screening substantially improves . . .