amniocentesis

amniocentesis , diagnostic procedure in which a sample of the amniotic fluid surrounding a fetus is removed from the uterus by means of a fine needle inserted through the abdomen of the pregnant woman (see pregnancy ). The procedure can be done in a hospital or in a doctor's office. Ultrasound is used to determine the location of the fetus during the procedure. Fetal cells in the fluid can be grown in the laboratory and studied to detect the presence of certain genetic disorders (e.g., Down syndrome , Tay-Sachs disease ) or physical abnormalities (e.g., anencephaly, or incomplete development of the brain). The sample also can be examined to determine the gender of the fetus and has been used to preselect the sex of the baby, a practice that, although controversial, is much used in some parts of the world. Amniocentesis cannot be used to detect such defects as congenital heart disease or cleft palate .

Amniocentesis is generally recommended when there is a family history of genetic disorders or when the woman is over age 35 and therefore at a higher risk of having a baby with a chromosomal abnormality. The procedure is usually carried out around the 14th or 15th week of pregnancy, when there is sufficient amniotic fluid and abortion is still an option. It can also be used in the third trimester (after 30 weeks) when Rh incompatibility (see erythroblastosis fetalis ) is suspected, or to determine the status of the fetus in early or late delivery or when there are signs of fetal distress.

See also birth defects ; chorionic villus sampling ; embryo biopsy .

amniocentesis Removal of fluid from the amniotic cavity. This is accomplished by passing a long, fine needle through the abdominal wall and through the underlying wall of the pregnant uterus, and aspirating a sample into a syringe. The purpose is to obtain stray cells shed from the fetus into the fluid, which can provide evidence of genetic abnormalities, especially Down's syndrome. Because the risk of conceiving such an infant increases with age, amniocentesis is commonly advised at about 16 weeks of pregnancy in any mother over about 35. The procedure is combined with ultrasound scanning, which allows the fetus, the placenta, and the needle to be visualized, and damage thus avoided. There is a very small risk of inducing miscarriage (less than 1% in good hands). Preliminary tests on the mother's blood may indicate how strongly amniocentesis is to be advised.

Stuart Judge