Amniocentesis is the most common invasive prenatal diagnostic procedure used for fetal karyotype testing. During the procedure, performed between the 15th and 20th weeks of pregnancy, a long needle is inserted through the mother’s abdomen and into the uterus to collect a small amount of the amniotic fluid that surrounds the baby. The sample is sent to a lab, where the baby’s cells are studied in order to diagnose or rule out prenatal conditions. An amniocentesis may be recommended in cases where there is an expected or increased risk of aneuploidy, genetic conditions, or open neural tube defects. An amniocentesis is an invasive procedure; it does come with risk. The risk for miscarriage following an amniocentesis is 1 in 200. Amniocentesis is considered less risky than chorionic villus sampling (CVS), another invasive prenatal diagnostic procedure.