Genetic Blood Test for Down Syndrome? Not Yet

But there’s a risk-free, reliable method that’s already available.

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Corrected on 10/07/08: An earlier version of this article incorrectly suggested that all pregnant women should get, rather than be offered, Down syndrome screening.

Pregnant women have been waiting a long time for a blood test to definitively diagnose Down syndrome, and one may finally be on the near horizon. Researchers report in the Proceedings of the National Academy of Sciences that they've overcome a significant hurdle in providing such a test, which won't require them to sort maternal genetic material from a fetus's. Another test manufactured by Sequenom could be ready as early as June, according to the New York Times.

Neither method, though, has been rigorously tested in enough women to ensure that it will predict, with absolute certainty, which babies carry the Down genetic defect and which ones don't. The disorder occurs in as many as 1 in 800 babies and causes mental retardation, facial abnormalities, heart defects, and other problems.

At this point, the two definitive ways to diagnose Down involve inserting a needle into a woman's abdomen to draw out fetal cells—both of which carry a small risk of miscarriage, about one half of 1 percent. But, as I previously reported, a combination test is already available that's about 95 percent reliable for identifying Down, without posing any risk of miscarriage.

The American College of Obstetricians and Gynecologists now recommends that all women in their first 12 weeks of pregnancy be offered both a blood test that checks for four Down markers. While negative test results are great for reassuring mothers, any result indicating that a baby has Down still needs to be confirmed using a needle-in-the-abdomen method, either amniocentesis or chorionic villus sampling. Clearly, a new genetic blood test would be great for pregnant women, but we may still be waiting a bit before one becomes available.