amniocentesis

Sampling the amniotic fluid surrounding a fetus in the womb for diagnostic purposes. It is used to detect Down's syndrome and other genetic abnormalities. The procedure carries a 1 in 200 risk of miscarriage.

A needle is inserted into the mother's abdominal wall and, usually under ultrasound guidance, a small amount of the amniotic fluid surrounding the fetus is withdrawn into a syringe.

This fluid, containing shed fetal cells, is sent for laboratory evaluation. In the laboratory, fetal cells can be cultured to provide a chromosomal profile of the unborn child (fetal karyotype), revealing any genetic abnormality such as Down's syndrome. Amniotic fluid can be tested to detect a growing number of conditions, such as spina bifida, which may result in severe disability.

Early amniocentesis (about the 16th week of pregnancy) is usually advised for all high-risk mothers: those over 35 years of age (who are most likely to have a Down's syndrome baby) and those with a family history of inherited disease.