Your Guide to Prenatal Tests

Expanded AFP Test (Triple Screen)

Who: Available to all women

When: Between 15 and 20 weeks, although safest between 16 and 18 weeks

How: A blood test

Why: This test checks for genetic abnormalities such as trisomies 18 and 21, neural tube defects (such as spina bifida), major brain defects (anencephaly) and abdominal wall defects (such as gastroschisis and omphalocele), as well as Smith-Lemli-Opitz Syndrome (a rare developmental disorder that affects many parts of the body and is characterized by small head size, intellectual disability, and behavioral problems). It measures the level of a protein produced by the fetus called alpha-fetoprotein (AFP) as well as that of two hormones, human chorionic gonadotropin (hCG) and estriol.

Results and follow-up: Results are available within one to two weeks. If your levels of AFP and estriol are low and the level of hCG is high, your baby has an increased risk of Down syndrome. A high reading of AFP may signify a neural tube defect. If your results are abnormal, you'll be offered an amniocentesis. Note: These tests have a high false-positive rate: 50 out of every 1,000 women will be told that their fetus is at risk; of these, only one or two babies will actually have Down syndrome.

Note: Although the triple screen is the most common test, there are three newer versions of it that your doctor may order instead: The quadruple screen, serum integrated screening, and full integrated screening.