A new study and more conservative federal guidelines may ease concern over the safety of chorionic villus sampling, the earliest prenatal test to detect fetal birth defects.
After the test was linked last year to a heightened risk of birth defects, its use dropped significantly.
However, the latest study, by a California physician who has performed thousands of such samplings, reinforces what some experts have said all along: Women who want chorionic villus sampling, or CVS, should use physicians who have done it thousands of times.
To reduce risk of birth defects, the U.S. Food and Drug Administration is advising physicians to wait until the 10th week of pregnancy to perform the test. Previously, it was done as early as the eighth week. The FDA also recommends that doctors tell patients about the risk of fetal limb defects.
The FDA recommendations respond to reports on the two studies published last year in medical journals. One was by Humana Hospital-Michael Reese of Chicago, the other by the University of Milan in Italy.
In the Italian study of 321 women, four infants were born with missing or deformed fingers and toes, and of those infants several also had shortened tongues, underdeveloped lower jaws or small birthmarks. The Humana study found four of 394 infants had malformed limbs. Both studies indicated a rate of limb defects well above that of the general population, 5.4 per 10,000 births, but other studies disputed that high incidence: Doctors at Illinois Masonic Medical Center in Chicago reviewed more than 80,000 CVS tests done worldwide and detected no link between the procedure and limb malformations.
Some doctors believe that the two smaller studies were flawed, in part because of the early testing of the women in the Italian study: Some of them had the procedure in the sixth or seventh week of pregnancy. (The Illinois Masonic study included only those procedures performed when the fetus was at least nine weeks old.) These doctors insist that CVS is safe when done by experienced doctors and that the FDA is overreacting.
But Marcia Meyer, an agency spokeswoman, says, “The FDA has to take a conservative approach; we’d be remiss if we didn’t.”
To perform the test, a doctor takes a sample of placental tissue with a catheter inserted through the uterus. Sometimes samples are taken by inserting a thin needle into the woman’s abdomen. The tissue comes from the chorion, part of the developing placenta. Fingerlike projections on the chorion, called villi, are suctioned away to provide cells for genetic testing.
The cells are grown in a laboratory and examined for chromosomal abnormalities that would indicate birth defects. Results usually are available within 10 days.
With amniocentesis, by comparison, the test usually is done at 16 to 18 weeks and results usually aren’t available until 4 1/2 to 5 months of pregnancy.
Sometimes, particularly when done as early as the 13th week of pregnancy, amniocentesis may not provide as much genetic data as CVS. So some experts consider CVS the procedure of choice for women over 35 and those with a history of genetic diseases, such as sickle-cell anemia, cystic fibrosis and some types of muscular dystrophy.
Both tests appear to carry the same risk of miscarriage that naturally occurs with pregnancy, about one in 1,000 procedures, although some studies attributed higher risk to CVS.
CVS grew in popularity since it first was performed in the U.S. in 1984. In recent years, some 30,000 to 40,000 women a year have had the test, about one-third the number who have chosen amniocentesis. But interest in CVS dropped significantly last year after the reports of limb defects.
The latest study, in the December issue of Obstetrics and Gynecology, found that a doctor’s proficiency improves with experience. If a doctor must take three samples to get enough material for the test, the risk of miscarriage is greater because the chance of miscarriage increases with each insertion.