CHICAGO --- First-trimester tests for two proteins in the blood of pregnant women, combined with ultrasonography measurements of fetal neck skin, may provide the earliest diagnosis yet of fetal birth defects in at-risk women. The combined-risk assessment method has an estimated detection rate of Down syndrome of about 90 percent and is completely non-invasive.
Northwestern University Medical School researchers are evaluating the combined-risk assessment method, factoring in duration of pregnancy and maternal age at the time of screening, to determine the test's value in predicting fetal abnormalities. The National Institutes of Health-sponsored study also will assess if such a screening system could be implemented nationwide.
"Overall, we may end up with an earlier test (first vs. second trimester), with a higher detection rate (90-plus percent vs. 60 percent), doing less-invasive procedures and losing fewer normal pregnancies than might have been lost as a direct consequence of invasive procedures," said Eugene Pergament, M.D., lead investigator on the study, who is a professor of obstetrics and gynecology at the Medical School and director of reproductive genetics at Northwestern Memorial Hospital.
Birth defects such as Down syndrome occur more commonly in pregnancies in women aged 35 and older. For women at advanced maternal age, two procedures, chorionic villus sampling (CVS) in the first trimester (first 12 weeks of pregnancy) and amniocentesis in the second trimester (12 to 24 weeks of pregnancy), are offered. The accuracy of CVS and amniocentesis is very close to 100 percent. However, both procedures carry a risk of losing the pregnancy because they are invasive.
CVS uses either a catheter placed in the placenta through the cervix or a needle placed in the placenta through the abdomen. Amniocentesis is similar to CVS because it is performed with a needle inserted through the abdomen.
"Some women do not undertake CVS or amniocentesis -- even if they are at increased reproductive risk for a conception with a genetic disorder -- because of their unwillingess to risk loss of the pregnancy as a direct consequence of performing either procedure," Pergament said.
Pregnant women under 35 can have a blood test in the second trimester that screens for spina bifida, Down syndrome and other chromosome and structural abnormalities in the fetus. Second-trimester screening in this age group can identify 80 percent of fetuses with spina bifida and about 60 percent with Down syndrome.
Recently, it became possible to screen for Down syndrome in the first trimester by measuring two proteins present in the blood of pregnant women. One of the proteins, human chorionic gonadotrophin (or free b hCG), is present in high concentrations in Down syndrome cases. The second, pregnancy-associated plasma protein A (or PAPP-A), is present in low concentration in Down syndrome cases. When combined, these two protein markers are estimated to yield a rate of detection of Down syndrome of 62 percent, similar to the screening test now commonly used in the second trimester.
Another screening technique became available that factors in maternal age and measures by ultrasonography the thickness of the fetal skin in the area of the neck (nuchal) measured at 10- to 14-weeks' pregnancy. Over 100 years ago, Dr. Down reported that the skin of affected individuals appeared to be too large for their bodies. It is now known that the excess skin of fetuses with Down syndrome can be visualized by ultrasonography in the first three months of intrauterine life.
Research suggests that screening by maternal age and thickness of skin at the neck now identifies more than 80 percent of affected fetuses. When the maternal blood proteins free b hCG and PAPP-A at 10- to 14-weeks' gestation also are taken into account, the detection rate of Down syndrome is estimated to be as high as 90 percent. Furthermore, increased neck skin thickness at 10- to 14-weeks' gestation is commonly associated with many other genetic disorders and chromosome abnormalities beside Down, as well with the presence of cardiac defects in fetuses.
All pregnant women who are at 10- to 14-weeks' gestation, who do not have twins, and have not had any bleeding during their current pregnancy are eligible to participate in the study. Before deciding whether to participate in the study, women should discuss the matter with a physician or a genetic counselor. A genetic counselor at Northwestern Memorial Hospital will be available for a free personal discussion. For information, call (312) 908-7441.
Contact: Elizabeth Crown
312-503-8928
e-crown@nwu.edu
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Stephanie Clemson
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