Amniocentesis is an extraction of amniotic fluid (the liquid surrounding the fetus in the uterus) with a needle. The needle is inserted through the abdomen and the uterine wall, after the pregnant woman has been given an injection of local anesthetic.
Amniocentesis is done if the doctors suspects of spina bifida, Down syndrome or some other anomaly. This possibility exists if you already have an abnormal child, if there is an anomaly in the family history, or if you have passed the age of 35. The procedure is usually performed between the 16th and 18th week of pregnancy.
First, ultrasound determines the exact position of the fetus and placenta. If test (using the amniotic fluid) show a high percentage of abnormalities, pregnant women are advised to stop pregnancy.