Genetic amniocentesis is a screening examination done during pregnancy to diagnose chromosomal abnormalities in the fetus. Usually an amniocentesis is done at between 16-20 weeks of pregnancy (gestation). During this procedure amniotic fluid is removed for testing. The amniotic fluid contains cells that the baby sheds naturally, from their skin, lungs and urinary tract. Cells and proteins within the amniotic fluid are examined in the lab to test for specific fetal disorders, such as chromosomal disorders like Down syndrome, Patau syndrome, and Turner syndrome. Alpha-fetoprotein (AFP) also found in the amniotic fluid and can be measured in the laboratory.

An amniocentesis takes approximately 45 minutes, the majority of which is during a detailed ultrasound. Ultrasound is also used during the procedure.