Amniocentesis is a procedure used to diagnose fetal defects in the early second trimester of pregnancy. A sample of the amniotic fluid which surrounds a fetus in the womb is collected through a pregnant woman's abdomen using a needle and syringe. Tests performed on fetal cells found in the sample can reveal the presence of many types of genetic disorders, thus allowing doctors and prospective parents to make important decisions about early treatment and intervention.

Since the mid-1970s, amniocentesis has been used routinely to test for Down syndrome, by far the most common genetic birth defect. By 1997, about 800 different diagnostic tests were available, most of them for hereditary genetic disorders such as Tay-Sachs disease, muscular dystrophy and cystic fibrosis.

Amniocentesis is recommended for women over age 35, or who have already borne children with birth defects, or when either of the parents has a...