Amniocentesis, am nee oh sehn TEE sihs, is a medical procedure performed during pregnancy to help determine the health and maturity of an unborn baby. It involves the withdrawal and study of a small amount of the amniotic fluid that surrounds the fetus in the mother's uterus. With this procedure, physicians can accurately diagnose more than 150 serious disorders that may affect the fetus. Such disorders include Down syndrome and Tay-Sachs disease. Amniocentesis involves little risk to either the mother or the fetus.