Screening pregnant women during their first trimester could identify more pregnancies affected by Down syndrome while reducing the number of invasive prenatal diagnostic tests, such as amniocentesis, a new study reports.

Screening in the first trimester with a blood test and ultrasound can identify 87 percent of fetuses with Down syndrome, concludes a National Institutes of Health-financed study of more than 38,000 U.S. women.

Adding information from another blood test in the second trimester can lead to detection of 95 percent of affected fetuses, but second-trimester screening alone picks up only 81 percent, researchers report in the New England Journal of Medicine. The rate of false positives, or fetuses that appeared to have Down syndrome but were found to be normal after further testing, was 5 percent.

Lead author Fergal Malone says the findings could greatly reduce the number of women who undergo chorionic villus sampling (CVS) or amniocentesis, invasive procedures that are the only prenatal tests that can conclusively diagnose Down syndrome. CVS is performed in the first trimester, amniocentesis in the second. In experienced hands, the tests are equally safe, Malone says, but both carry a small risk of infection and miscarriage.

The risk of Down syndrome rises with the mother's age, which prompted an NIH panel in 1978 to recommend that all pregnant women 35 and older be offered amniocentesis. Yet, according to the March of Dimes, 80 percent of Down syndrome babies are born to women under 35, because they have far more babies than older women.

Offering amniocentesis only to women 35 and older diagnosed only 25 percent to 30 percent of pregnancies affected by Down syndrome, Baylor College of Medicine OB/GYN Joe Leigh Simpson writes in an accompanying editorial. The introduction of second-trimester screening in the 1980s raised that rate to 60 percent to 70 percent, Simpson writes.

Malone, who is on the faculty of the Royal College of Surgeons in Dublin, says he has been offering first-trimester screening to patients of all ages for three years. He tells each patient her individual risk of carrying a fetus with Down syndrome.

"If you got an extremely reassuring number — 1 in 2,000, 1 in 5,000, which a lot of women do — that may be all the reassurance you want," Malone says. "I would very much be against saying just because you're over 35 you should have an amnio. It is still more efficient to do screening first, even in the older age group."

Most OBs offer second-trimester screening, but Malone says first-trimester screening is not yet widely available. Medicaid and many health plans don't yet cover it, he says, adding that he hopes the publication of his findings will change that.

And for now, few doctors outside of academic medical centers know how to perform a key part of first-trimester screening, Malone says. Besides a blood test to measure two substances linked to Down syndrome, screening requires a tricky ultrasound scan to measure nucal translucency, the tiny fluid-filled space at the base of a fetus's neck, which tends to be larger in fetuses with Down syndrome.

Sources: March of Dimes and the National Down Syndrome Society