A test for Down's syndrome that can be done a full month earlier than the standard method of finding fetal abnormalities has proved safe in a study of more than 150,000 women, researchers reported Thursday.
Some research had suggested the test, chorionic villus sampling, or CVS, could slightly increase the risk of birth defects. The study found no evidence of that.
Amniocentesis, the standard test, does not provide results until the 14th to 16th week of pregnancy. CVS provides results in the 10th week of pregnancy.
The findings are important because demand for the test, which is now available in many large research hospitals, is likely to grow as other advances in genetics increase its usefulness, said one of the study's authors, Dr. Laird Jackson of Thomas Jefferson University in Philadelphia.
Despite Jackson's findings, U.S. health officials will issue a report soon saying they think there is a very slight increase in limb abnormalities in fetuses that have undergone the procedure.
"The risk is still low. I guess that's the good news," said Dr. Muin Khoury of the birth defects branch at the Centers for Disease Control and Prevention in Atlanta.
One explanation for the disagreement is that the CDC study may have included doctors less skilled at performing CVS.
The CDC data, collected from birth-defects records in seven states, suggests that the risk of limb deformities is six times higher than normal in fetuses that have undergone the procedure.
But because the normal risk of such abnormalities is only one in 18,000 cases, the risk after CVS is still low, occurring in about one in 3,000 cases, Khoury said.
The risk may be outweighed by the psychological relief associated with knowing a month earlier that the fetus is not afflicted with Down's or other chromosome abnormalities, Khoury said.
Chorionic villus sampling is done by removing a sliver of tissue from the chorion, the tissue that gradually develops into the placenta. This tissue is then examined under the microscope for evidence of an extra copy of chromosome 21, the mark of Down's syndrome, or for other chromosome abnormalities.
In amniocentesis, a needle is used to draw out a sample of the amniotic fluid surrounding the fetus. The fluid contains bits of fetal tissue.
Chromosome testing is recommended for women who become pregnant after age 35 because the risk of Down's syndrome increases with the mother's age.