A diagnostic procedure for detecting abnormalities of the FETUS. Usually carried out between the 16th and 18th week of pregnancy, amniocentesis is performed by piercing the amniotic sac in the pregnant UTERUS with a hollow needle, under ULTRASOUND guidance and withdrawing a sample of AMNIOTIC FLUID for laboratory analysis. As well as checking for the presence of abnormal fetal cells, the procedure can show the sex of the fetus. The risk of early rupture of the fetal membranes or of miscarriage is low (around 1 per cent). Currently it is offered (in the UK) to women over age 35, those who have previously had a baby with SPINA BIFIDA, CHROMOSOME abnormality and those known to carry a serious genetic disorder. It is also offered to women who have had an abnormal result on fetal ultrasound screening or blood testing for DOWN'S SYNDROME.