Earlier Testing for Down Syndrome
Jan. 17, 2002 -- For the time being, if a pregnant woman decides she wants to have her fetus tested for Down syndrome, she must wait until the second trimester. But that may soon change. Research sponsored by the National Institutes of Health shows that a new approach can provide more accurate information much earlier in pregnancy.
The researchers presented their findings today at the Society for Maternal-Fetal Medicine annual meeting in New Orleans.
Many specialists recommend that women who become pregnant at age 35 or older undergo prenatal testing for Down syndrome. The likelihood that a woman under 30 who becomes pregnant will have a baby with Down syndrome is less than 1 in 1,000, but the risk jumps to 1 in 400 for women who become pregnant at age 35, according to the National Institutes of Health. The likelihood goes up from there: 1 in 60 chance by age 42; 1 in 12 chance by age 49.
Currently, prenatal testing for Down syndrome usually involves amniocentesis, done between 14 and 18 weeks of pregnancy. In this procedure, a needle is inserted into the mother's womb and a small amount of fluid that surrounds the fetus is withdrawn for chromosomal testing. Another option is chorionic villi sampling (CVS), which can be done earlier, from 9 to 11 weeks. Again, a needle is passed into the womb, but in this test, a small amount of tissue that makes up part of the placenta is removed.
In the current study, Ronald J. Wapner, MD, and colleagues from Philadelphia's MCP Hahnemann University tested more than 8,500 women, average age 34 years, who were between weeks 10 and 12 of pregnancy.
The new approach combines tests for certain biological markers in the blood -- PAPP-A and hCG -- and ultrasound measurement of what's known as nuchal translucency, which is the thickness of skin behind the fetus' neck.
Taken together with the risk conferred by the mother's age, these measures detected with 85% accuracy whether a fetus had the chromosomal abnormality that causes Down syndrome. According to the researchers, this is a significant improvement over current screening methods, which identify only 65% of cases and falsely alarm about 5% of women.
The tests used for detecting Down syndrome have a slight chance of causing miscarriage. Women should talk with their doctors before deciding to undergo amniocentesis or CVS.