New Screening Detects Birth Defects Early
Battery of noninvasive tests effective in first trimester, study finds
WEDNESDAY, Oct. 8, 2003 (HealthDayNews) -- A battery of tests in the first three months of pregnancy can catch most cases of two birth defects, Down syndrome and trisomy 18.
The finding should help settle a debate about whether such early screening is as effective as testing in the second trimester, says study leader Dr. Ronald Wapner, chairman of obstetrics and gynecology at Drexel University College of Medicine.
"It strongly suggests that screening women in the first trimester is a very viable alternative to waiting," Wapner says.
The study, reported in the Oct. 9 issue of the New England Journal of Medicine, combined a blood test for two telltale molecules, human chorionic gonadotropin and pregnancy-associated plasma protein A, with an ultrasound scan to detect 85 percent of cases of Down syndrome (also known as trisomy 21) and 90 percent of trisomy 18 cases in more than 8,500 first-trimester pregnancies.
Positive results on the screening tests must then be confirmed by what are called invasive tests, either by amniocentesis, taking a sample of fluid from the sac that contains the fetus, or by a biopsy of tissue from the placenta. Both procedures involve risks. The advantage of the early screening tests is that they let most women skip that risk.
"This is far and away the largest study to look at first-trimester screening," Wapner says.
The results of the study, the largest of its kind in the United States, are similar to those of several others, including a British trial of more than 30,000 pregnancies.
Down syndrome, which causes mental retardation, is much more common than trisomy 18, which usually is fatal to the fetus. Both defects become more common in older mothers-to-be.
But more work needs to be done, says another expert. "What is needed is a head-to-head comparison of those two approaches in the same set of pregnancies," says Dr. Fergal Malone, an associate professor of obstetrics and gynecology at the Columbia University College of Physicians and Surgeons. "Whichever is shown to be better would change the standard of care."
That study has been done, with Malone as one of its leaders. More than 38,000 pregnant women got first-trimester screening of the sort in the latest study, followed by second-trimester screening that measured blood levels of four substances: human chorionic gonadotropin, alpha-fetal protein A, inhibin A and estriol.
Those results will be presented at a February meeting of the Society for Maternal-Fetal Medicine and will have a major influence on how screening is done, Malone says.
Screening is of growing importance because the incidence of birth defects such as Down syndrome increases with age, and "more women are looking to start families later," says Dr. Deborah A. Driscoll, an associate professor of obstetrics and gynecology at the University of Pennsylvania School of Medicine and co-author of an accompanying editorial.
The new study is valuable because it shows that first-trimester screening can be effective in early detection of birth defects, Driscoll says. "The earlier we can learn about abnormal pregnancies, the more desirable it may be for some women," she adds.
The report being prepared by Malone and his colleagues will help obstetricians decide whether screening during the first trimester, the second trimester or both trimesters is most effective, Driscoll says.
The American College of Obstetrics and Gynecology recommends screening for all pregnant women, regardless of age. In practice, Malone says, many older mothers-to-be "go straight to amniocentesis. More and more women are doing it."