Clinical implications of first-trimester screening.

First-trimester aneuploidy screening is associated with very high detection rates for Down syndrome and other chromosomal abnormalities. The clinical implications of early screening relate to prenatal diagnosis of chromosomal abnormalities and important information obtained at screening. Early screening can change the rate of invasive prenatal diagnosis and the gestational age of prenatal diagnosis and abortion, and lead to earlier prenatal diagnosis of certain major structural abnormalities. The detection of biochemical abnormalities associated with obstetric complications, such as low pregnancy-associated plasma protein A, could also affect the management and outcomes of some pregnancies.