Contemporary approaches to prenatal diagnosis.

A variety of options for prenatal diagnosis are available to the pregnant woman. Maternal serum analyte analysis, performed between 15 and 20 weeks' gestation, provides a screening test for fetal neural tube defects and aneuploidy in low-risk pregnancies. Fetal ultrasound examination is of benefit in high-risk pregnancies. The necessity of prenatal screening in the low-risk patient with an established date of last menstrual period is more controversial. Ultrasound examination can establish gestational age, assess fetal number and position, determine placental location and amniotic fluid volume, and rule out major structural anomalies. Invasive fetal testing, including amniocentesis and chorionic villus sampling, should be offered to women who are 35 years of age or older or who have had abnormal results on noninvasive prenatal screening and in cases in which the parents are carriers of genetic conditions that are amenable to prenatal diagnosis.