What to Expect From Prenatal Genetic Testing

Indeed, most of the time, the fetus is not affected with the disorder, even if the screening result is abnormal. Out of 1,000 serum screening tests, an average of 50 will suggest an increased risk for open neural tube defects, but only one or two of the fetuses will have such a defect. Likewise an average of 40 out of 1,000 will test positive for increased risk of Down syndrome, but only one or two fetuses will actually have the condition.

Most labs offer the standard "quadruple marker" serum screening test, which detects approximately 75 percent of fetuses affected with Down syndrome among women younger than age 35 and 90 percent of affected fetuses in women 35 and older.

Nuchal translucency screening

A newer type of prenatal screening test that can be done in the first trimester is becoming much more common, although it's still not offered everywhere. This test assesses the risk for the fetus to have Down syndrome or trisomy 18. The first trimester screening has two parts and both should both be performed between weeks nine and 13 of gestation. One part tests levels of maternal serum free beta-human chorionic gonadotropin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A) in the mother's blood. The other measures the nuchal thickening (measure of fetal neck thickness) through a specialized ultrasound. This test detects 85 percent of fetuses with Down syndrome and approximately 90 percent with trisomy 18.