Pregnancy: first-trimester screening - the best for Down syndrome detecting

Birth defects occur in about 4 percent of all babies born each year in the United States, according to the March of Dimes. One of the most common genetic causes is Down syndrome, a chromosomal birth defect that causes a flat facial profile, low muscle tone and varying levels of mental retardation.Researchers say that by using a test called first-trimester combined screening, they can now detect Down syndrome earlier in pregnancy than ever before. The combined screen includes an ultrasound test called a nuchal translucency test, plus tests that look for two proteins in maternal blood, according to Forbes.

Malone was lead author of the study, which compared the new test to standard second-trimester screening, called quadruple screening. His team published their findings in the Nov. 10 issue of the New England Journal of Medicine.

Second-trimester quadruple screening is currently the standard Down syndrome screening test used in the United States, according to the study. This test, which focuses on maternal blood proteins, is generally performed 15 weeks to pregnancy. If test results are abnormal, many women choose to undergo amniocentesis, which can diagnose Down syndrome. This means that a woman is often 18 or 19 weeks pregnant when she gets her amniocentesis results, Malone said.

He said the nuchal translucency test was first developed in the early 1990s and can only be done at between 11 and 13 weeks' gestation. Using ultrasound, a technician measures the thickness of the back of the fetus' neck. A difference of just a fraction of a millimeter, along with the presence of two proteins - pregnancy-associated plasma protein A and the free beta subunit of human chorionic gonadatropin - can signal the presence of Down syndrome.

If this combined screening test is positive, many women follow-up with chorionic villus sampling (CVS), which like amniocentesis can diagnose Down syndrome, but does so at an earlier stage of pregnancy. The optimal time for this test is 11 weeks. Before then, the fetus is too small, and after 13 weeks, the fetus is too large for the ultrasonographer to get an accurate picture.

To compare combined screening to standard quadruple screening, Malone and colleagues from centers in the United States, Ireland and the U.K. performed both tests on more than 38,000 pregnant women. One hundred and seventeen women in this group had babies with Down syndrome.

Both tests had an overall 5 percent false-positive rate, meaning that they picked up "cases" of abnormality that were in fact normal. The first-trimester combined screening was more effective in detecting Down syndrome, the researchers report.

Screening tests, which are based on complicated projections based on the blood tests and ultrasound, do not tell a women definitively if her baby has Down syndrome — it only gives the probability, ABC reports. A.M.