Amniocentesis is a procedure which involves extracting amniotic fluid from the womb to identify any abnormalities in the baby's chromosomes.
It is usually carried out between 15-20 weeks of pregnancy. Sometimes the procedure is done again between 18-22 weeks to check the risk of the foetus developing rhesus disease. In some cases it is necessary to monitor the foetus by carrying out further tests throughout the pregnancy.
Amniocentesis is generally avoided in the first 15 weeks of pregnancy as it can increase the risk of miscarriage and club foot in the baby.
Another diagnostic test which involves extracting tissue from around the placenta rather than amniotic fluid, is able to be performed from about ten weeks. This type of test is called chorionic villus sampling (CVS) and involves the same amount of risk as amniocentesis.
May 2011 | 
Amniocentesis Introduction
