A diagnostic procedure in which a small amount of amniotic fluid is withdrawn, using a syringe and guided by ultrasound scanning, from the amniotic sac that surrounds the fetus in the uterus. This fluid contains fetal cells that can be subjected to chromosome analysis to identify chromosomal defects such as Down’s syndrome or genetic analysis to look for genetic disorders such as haemophilia, cystic fibrosis, and Tay–Sachs disease. Chemical analysis of amniotic fluid can help to diagnose developmental abnormalities such as spina bifida. Rhesus incompatibility and maturity of the fetal lungs can also be checked. Amniocentesis is usually performed in the 14th–18th week of pregnancy. It slightly increases the risk of miscarriage or early rupture of the membranes and is therefore recommended only when the fetus is thought to be at increased risk of an abnormality. (See also antenatal care, chorionic villus sampling.)


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