Amniocentesis is a diagnostic procedure in which a small amount of amniotic fluid is withdrawn, using a syringe guided by ultrasound scanning, from the amniotic sac (the membrane that surrounds the fetus in the uterus).

Why it is done

The amniotic fluid contains fetal cells, which can be subjected to chromosome analysis in order to identify or exclude chromosomal defects, such as Down’s syndrome, or genetic analysis to look for genetic disorders such as haemophilia, cystic fibrosis, and Tay–Sachs disease. The amniotic fluid also contains chemicals; and analysis of the fluid can help to diagnose or exclude developmental abnormalities such as spina bifida. The severity of rhesus incompatibility and the maturity of the fetal lungs can also be checked by amniocentesis.

How it is done

Amniocentesis is usually performed in the 14th to 18th week of pregnancy. It may be performed earlier, but this is technically more difficult a this early stage. The skin of the abdomen is cleaned and a needle is inserted into the amniotic sac; (in all cases, ultrasound scanning is used to avoid contact with the fetus and placenta). An attached syringe removes some fluid for analysis.


Amniocentesis slightly increases the risk of miscarriage or early rupture of the membranes, and the procedure is therefore recommended only when the fetus is thought to be at increased risk of an abnormality.