Tests enable early detection of Down syndrome

Down syndrome can be safely detected in the first trimester of pregnancy – several weeks earlier than is now possible – using a combination of blood tests and ultrasound, researchers report today in the New England Journal of Medicine.

A study involving more than 38,000 pregnant women showed that the combination test could detect 87 percent of Down cases as early as 11 weeks into the pregnancy. The blood test now used in the second trimester detects about 80 percent of cases.

“These results will undoubtedly change national practice,” said Dr. Mary E. D’Alton of Columbia University College of Physicians and Surgeons, one of the study’s authors.

Early screening would give women carrying an affected embryo the option of having an abortion earlier in the pregnancy when the risks to the mother are lower, or allow them more time to prepare themselves should they carry the pregnancy to term.

Officials at the American College of Obstetricians and Gynecologists indicated Wednesday that they would probably update their guidelines on prenatal screening in light of the findings.

But experts cautioned that a limited number of imaging experts have been trained to perform the ultrasound test, which is trickier than a conventional ultrasound used, for example, to observe gross defects or to learn the fetus’ sex.

Down syndrome, which occurs in one of every 660 to 800 pregnancies, is one of the leading causes of mental retardation. Children born with the syndrome have distinctive facial features – eyes that slant upwards; small ears, nose and mouth; and a flattened nasal bridge – and other birth defects and medical problems. The problem is most often associated with pregnancy in older women.

The syndrome is caused by the duplication of one member of chromosome pair 21, a condition called trisomy 21. It can be verified with 100 percent accuracy by two prenatal tests: chorionic villus sampling or amniocentesis. Both procedures require removing cells from the fetus for testing and entail a small risk of producing a spontaneous abortion. Screening tests are used to identify high-risk pregnancies, minimizing the number who undergo the more invasive tests.