Amniocentesis is a procedure used to obtain amniotic fluid for prenatal diagnosis of a fetus. Cells naturally are exfoliated from the surface of the fetus and some of these cells survive for a time in the fluid surrounding the fetus in the amniotic cavity. Soluble biochemical material of clinical significance produced by the fetus may also accumulate in the amniotic fluid. The fluid can be analyzed for these substances directly. A local anesthetic is given and a hollow needle is inserted through the mother's abdominal wall into the amniotic cavity and a small sample of the fluid is withdrawn with a syringe attached to the needle. In order to insure the safety of the fetus, the procedure is monitored via an ultrasound scan. Viable cells in the fluid are then cultured (grown) in vitro. The chromosomes of the cultured cells can then be examined. Viewing the chromosomes under...