Amniocentesis

Amniocentesis, in medicine, a procedure for withdrawing a sample of the amniotic fluid that envelops the developing human fetus in the uterus. Fetal cells obtained from the fluid are cultured in the laboratory for indications of chromosomal abnormalities, defective enzymes, or deficient growth. Amniocentesis is usually performed during the fourth month of pregnancy in conjunction with genetic counseling of parents with suspected inheritable defects, when the pregnant woman's age might predispose the fetus to genetic abnormalities, or when the woman has been exposed to substances that could seriously harm fetal development.

After injection of a local anesthetic into the abdominal skin, a very fine needle is inserted through the abdomen and the uterine wall into the amnion, the sac that contains the amniotic fluid. About 30 ml (about 1 oz) of fluid is removed by suction for analysis. The procedure has been shown to increase the risk of subsequent miscarriage by 0.5 percent.

More than 75 abnormalities and the sex of the fetus can be ascertained by analyzing the fetal cells in amniotic fluid. Perhaps the most common use of the test is to detect Down's syndrome. See also Birth Defects.