Amniocentesis (am-nee-o-sen-TEE-sis) is a pre-natal outpatient diagnostic test that's used to determine the possible presence of birth defects, such as chromosomal abnormalities and certain types of inherited diseases. It may also be used to evaluate development of the fetus. The procedure involves testing a sample of amniotic fluid, the watery substance surrounding the baby. An amniocentesis is usually performed between the fifteenth and seventeenth week of pregnancy. However, it's now been safely performed at 10 to 14 weeks. 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, or had a blood test that indicates an increased risk of certain types of genetic disorders. Tests on the amniotic fluid and its cells 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. Keep in mind that only the suspected disorder is tested for; the process will not automatically look for all birth defects. Before beginning, the doctor determines the position of the baby by using an ultrasound. A local anesthetic is then used to numb the skin, and a needle attached to a syringe is inserted through the abdomen into the amniotic sac to withdraw about one-half ounce of fluid. This fluid is analyzed to determine if a possible defect exists. For more information on amniocentesis, contact a healthcare provider.