Your Guide to Prenatal Tests

Amniocentesis

Who: Women over 35, those who have—or whose partners have—a family history of genetic diseases, and those whose triple screen test results are abnormal

When: At 15 to 20 weeks

How: Guided by ultrasound, a doctor inserts a needle into the abdomen to draw a sample of amniotic fluid. Most women report it’s not as uncomfortable as it sounds.

Why: An amnio detects neural tube defects, Down syndrome, and other genetic disorders such as cystic fibrosis and thalassemia.

Results and follow-up: The test is 99 percent accurate; results are available in seven to ten days. A faster test, known as "FISH" (fluorescent in situ hybridization), can return results sooner, in two or three days. The miscarriage rate for the procedure is about one in 400.