What to Expect From Prenatal Genetic Testing

• An open neural tube defect. When the neural tube fails to close, the baby is born with an opening in the head (anencephaly) or spinal cord (spina bifida). Babies with anencephaly are stillborn or die soon after birth; those with spina bifida need surgery and may be paralyzed. The severity of spina bifida can vary greatly. One way to reduce your risk of having a child with a neural tube defect is to take folic acid (400 micrograms per day) before conception.
• Down syndrome (also called trisomy 21). An extra copy of chromosome 21 causes Down syndrome, characterized by mental retardation, certain facial features and sometimes heart defects.
• Trisomy 18. An extra copy of chromosome 18 causes this syndrome, which usually proves fatal during the first year of the baby's life and is associated with severe mental retardation.

The blood screen does not look directly at genetic material but instead measures four substances—alpha-fetoprotein, unconjugated estriol and human chorionic gonadotropin and Inhibin A—to determine whether you are at increased risk of having a baby with one of these disorders. A key fact to remember is that this test does not diagnose the disorders—it only screens for the substances and shows if there is an increased risk of a disorder. Further testing is always suggested to make a diagnosis.

Abnormal screening tests don't mean your baby is affected