Clinical proceduress in prenatal diagnosis.

The prenatal diagnosis of fetal genetic disease has become a routine part of obstetric care. Pregnancies at risk are identified by a number of factors, including maternal age, positive serum screening, a history of a previous affected child, a parental chromosome rearrangement or an ultrasound-identified anomaly. Invasive diagnostic testing can be performed in the first trimester by chorionic villus sampling or in the second trimester by amniocentesis. Both procedures are safe, with an equivalent 0.5% risk of procedure-induced pregnancy loss. When performed prior to the routine sampling window of 15 weeks, amniocentesis may increase the risk of talipes equinovarus, the highest risk being encountered prior to 13 weeks' gestation. When chorionic villus sampling is performed prior to 9 weeks' gestation, there may be an increased risk of limb reduction defects. The laboratory analysis of both procedures is reliable. Chorionic villus sampling has a 1-2% incidence of confined placental mosaicism, requiring additional evaluation in some cases.