which prenatal test samples the fluid that surrounds the fetus?
During an amniocentesis a small amount (usually 2 tablespoons) of amniotic fluid that surrounds the fetus is removed. The amniotic fluid contains cells that have been shed by the fetus during normal development. These cells may then be used for chromosome tests and/or specific genetic tests. The fluid itself is tested for the level of alpha feto-protein (AFP) or for biochemical genetic disorders, if appropriate. Amniocentesis is best done after 15 weeks and up to 23 weeks of pregnancy. The amniocentesis procedure involves guiding a thin needle through the mother's abdomen and into the amniotic sac (see below). Ultrasound is used to determine the location of the fetus and the best place to withdraw the fluid.