Both amniocentesis and CVS (chorionic villussampling) are diagnostic tests for chromosomal and genetic disorders such as Down syndrome and cystic fibrosis. Amnio also can detect neural tube defects such as spina bifida. About two or three of every hundred babies is born with such a defect, and most occur in the first three months of gestation. Because of the slightly increased miscarriage risk (more about this later), neither test is offered routinely. But, if your baby is at increased risk of chromosomal or genetic defects or earlier screenings suggest potential problems, expect to discuss the option of testing with your doc.
American College of Obstetrics and Gynecologists. Your pregnancy and birth. 4th ed. Washington, DC: ACOG; 2005.