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 (also referred to as amniotic fluid test or AFT), is a medical procedure used in prenatal diagnosis of genetic abnormalities and fetal infections , in which a small amount of amniotic fluid, which contains fetal tissues, is extracted from the amnion or amniotic sac surrounding a developing fetus, and the fetal DNA is examined for genetic abnormalities.


Before the actual procedure, a local anesthetic is sometimes given to relieve the pain when inserting the needle used to withdraw the fluid. A needle is usually inserted through the mother's abdominal wall or at the end of the vagina, and through the wall of the uterus into the amniotic sac. With assistance from ultrasound, a physician aims towards an area of the sac that is away from the fetus and extracts a small amount of amniotic fluid for testing. The puncture heals, and the amniotic sac replenishes the liquid over a day or so. After the amniotic fluid is extracted, the fetal cells are separated from it using a centrifuge, and the fetal chromosomes are examined for abnormalities. Various genetic testing may be performed, but the three most common abnormalities tested for are Down syndrome, Trisomy 18 and spina bifida. Amniocentesis can be performed as soon as sufficient amniotic fluid surrounds the fetus to allow a sample to be recovered relatively safely, usually no earlier than the 14th week of pregnancy. Often, genetic counseling is offered in conjunction with amniocentesis.


Although the procedure is routine, 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. Other possible complications include preterm labor and delivery, respiratory distress, postural deformities, fetal trauma and alloimmunisation (rhesus disease). The risk of amniocentesis-related miscarriage is generally thought to be 1 in 200, although a recent study has indicated this may actually be much lower, perhaps 1 in 1,600. In contrast, the risk of miscarriage for chorionic villus sampling (CVS) is believed to be approximately 1 in 100, although CVS may be done up to four weeks earlier, and may be preferable if the possibility of genetic defects is thought to be higher .

New knowledge has also been discovered in blood sampling. Blood can be extracted from the mother, and then tested

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