Amniocentesis is a medical procedure used in the prenatal diagnosis of chromosomal abnormalities and fetal infections, in which a small amount of amniotic fluid, which contains fetal tissues, is extracted from the amniotic sac surrounding a developing baby and the fetal DNA is examined for genetic abnormalities.

A needle is inserted through the mother’s abdominal wall, then through the wall of the uterus, and finally into the amniotic sac. With the aid of ultrasound guidance, a physician punctures the sac in an area away from the fetus. and extracts approximately 20 milliliters of amniotic fluid. After the amniotic fluid is extracted, the fetal cells are separated from the sample. The fetal cells are grown in a culture medium. Under microscope, the chromosomes are examined for abnormalities. In regard to the baby, the puncture heals and the amniotic sac replenishes the liquid over the next 24 to 48 hours.

Amniocentesis is performed between the 16th and 20th week of pregnancy. Most people do amniocentesis during the 18th week of pregnancy.

Possible complications include infection of the amniotic sac from the needle, and failure of the puncture to heal properly, which can result in leakage or infection. Serious complications can result in miscarriage. Amniocentesis carries a risk of miscarriage often due to the rupture of the amniotic sac. The risk of amniocentesis is highest when the procedure is done early in pregnancy, before the two layers of fetal membranes have sealed. A recent study (2006) estimated the risk of amniocentesis related to miscarriage as low as 1 in 1,600.