Amniocentesis (am-nee-o-sen-TEE-sis) is a pre-natal outpatient test used to determine the possible presence of birth defects, or to evaluate fetal maturity. It's usually performed between the 15th and 18th weeks of pregnancy. Your doctor may suggest an amniocentesis if you're 35 years or older, have had a previous child with birth defects, have a family history of genetic disorders, are scheduled for a cesarean birth, or are expected to have premature labor. Tests on the amniotic fluid and cells in the fluid can detect birth defects such as Down's syndrome, spina (SPINE-a) bifida (BIFF-ih-da), some forms of muscular dystrophy, and some brain and blood disorders. During the procedure, the doctor determines the position of the baby by using an ultrasound. A local anesthetic is sometimes used to numb the mother's abdomen, and a needle attached to a syringe is inserted through the abdomen and uterus into the amniotic sac, withdrawing about one ounce of fluid. This fluid is analyzed to determine if there are any possible defects. Though the risk of complications is quite low, it does exist, so the procedure should only be performed when there's good cause. For more information on amniocentesis, contact a health care provider.