Who: Some women over age 35 because of an increased risk of Down syndrome and other genetic conditions. Women who have had a previous child or pregnancy with a birth defect. Women who have a family history of a genetic disorder. Women who need further evaluation because of the results of first-trimester screening.

Why: To test for certain birth defects.

When: 10-12 weeks after the pregnant woman's last menstrual period.

How: There are two ways to perform the test. With both of them, the health care provider uses ultrasound as a guide. The provider may insert a thin tube through the vagina and cervix into the uterus. Or he or she may insert a thin needle through the wall of the belly into the uterus. With both methods, the provider takes a tiny tissue sample from outside the sac where the baby develops.

Risks: If the provider is experienced, CVS and amniocentesis have about the same risk of miscarriage (1 in 200 or less). Some women have cramping or spotting for a few hours after the procedure. Call your health care provider if you have heavy bleeding, fever or contractions.

April 2007