Just 11 weeks after conception, a pregnant woman can have reliable screening tests to look for signs of Down syndrome in the baby she is carrying. This news comes from an article in the November 10, 2005 issue of the New England Journal of Medicine. The article describes a study of 38,000 women known as the FASTER Trial (First and Second Trimester Evaluation of Risk), which was sponsored by the U.S. National Institute of Child Health and Human Development.

The study found that a combination of an ultrasound and blood tests helps pregnant women learn earlier and more reliably if their fetus is likely to have Down syndrome. The combination test is best at 11 weeks. Even at 13 weeks, the test is as reliable as the screening method most often used currently; the advantage is that the ultrasound/blood test combination gives information weeks earlier. (After 13 weeks, however, other tests must be used.) In the study, screening at 11 weeks identified 87% of the fetuses with Down syndrome. When combined with the right schedule and mix of follow-up tests, this approach was 96% effective in identifying fetuses with Down syndrome.

The study compared the first trimester tests to the screening method most often used in the U.S.: a set of lab tests done during the second trimester, during the 15th to 18th week of pregnancy, which they found to be only 81% effective. The second-trimester test, known as the quadruple screen, measures levels of four substances in the mother’s blood: alpha fetoprotein (AFP), unconjugated estriol (uE3), human chorionic gonadotropin (hCG), and inhibin-A. Both tests factor in the mother’s age because risk increases with age. In both approaches, false positive results occurred 5% of the time. Women pregnant with more than one child—twins, triplets, etc.—were not studied.

After the first-trimester test, women with results of concern are able to have definitive diagnostic tests sooner. The genetic tests used as confirmation—such as chorionic villus sampling, which can be performed at 12 weeks, or amniocentesis, which can be performed at 15 weeks—are considered invasive medical procedures because they require a tissue or fluid sample from the placenta. They are riskier for the fetus and mother, but allow the fetus’s own chromosomes to be checked for abnormalities. A drawback of the second-trimester test is that amniocentesis results may not be known until the 18th or 19th week of pregnancy.

The noninvasive screening method that performed best and can be done earliest uses an ultrasound imaging exam, called a nuchal translucency test, to measure the width of the fetus’ neck and a set of two lab tests run on a sample of the mother’s blood. The tests of the mother’s blood look for unusual levels of two proteins coming from the placenta or fetus: pregnancy-associated plasma protein A and the free beta subunit of hCG. The researchers note that the ultrasound test is difficult to perform and requires expertise that may not, as yet, be widely available. Many major medical centers in the U.S. and Europe, however, offer the combined screening test.

For most women, early assurance that a fetus is most likely not at risk for Down syndrome reduces anxiety and allows her to avoid invasive procedures and their health risks. Based on the results of this study, the American College of Obstetricians and Gynecologists is planning to update its recommendations on prenatal screening, reports the Washington Post. The U.S. government’s National Women’s Health Information Center notes that not all healthcare providers have access to this ultrasound exam, but those who use the test see it as a good option for many women. The ultimate goal in prenatal testing is a noninvasive test that is both simple and definitive—a true diagnostic test, not just a screening test. For now, though, women who are concerned about having a child with Down syndrome have a more compelling option they can pursue in the early months of pregnancy.

Sources
S1
National Down Syndrome Society. Are any prenatal tests available to detect Down syndrome? Available on the Internet at http://www.ndss.org/content.cfm?fuseaction=InfoRes.Generalarticle&article=30. Accessed 2 Dec 2005.

S2
National Women’s Health Information Center. Healthy pregnancy: the second trimester: screening tests. U.S. Department of Health and Human Services, Office on Women’s Health. Available on the Internet at http://www.4woman.gov/Pregnancy/second.cfm#8. Accessed 30 Nov 2005.

S3
Stein R. Down syndrome now detectable in 1st trimester: earlier diagnosis allows more time for decisions. Washington Post. 10 Nov 2005. A 01.