ACOG Committee Opinion #296: first-trimester screening for fetal aneuploidy.

First-trimester screening for chromosomal abnormalities offers potential advantages over second-trimester screening. Studies in the 1900s demonstrated an association between chromosomal abnormalities and the ultrasonographic finding of abnormally increased nuchal translucency (an echo-free area at the back of the fetal neck) between 10 and 14 weeks of gestation. First-trimester screening using nuchal translucency, free beta-hCG, and pregnancy-associated plasma protein-A has comparable detection rates and positive screening rates for Down syndrome as second-trimester screening using 4 serum markers (alpha-fetoprotein, beta-hCG, unconjugated estriol, and inhibin-A). Although first-trimester screening for Down syndrome and trisomy 18 is an option, it should be offered only if certain criteria can be met.