CHICAGO, DEC. 24 -- Placental sampling is a promising new method of detecting genetic abnormalities early in pregnancy, but it has not yet been proven effective and questions remain about its safety, a panel of medical specialists has concluded.

However, the panel assembled by the American Medical Association said studies are already under way to determine whether the technique, called chorionic villus sampling (CVS), is as safe as amniocentesis, a procedure commonly used to determine abnormalities about 15 weeks into a pregnancy.

If the safety questions are answered satisfactorily, CVS could prove a major advance because it can be done as early as the ninth week and theoretically should produce better results than the fluid sampling method that constitutes amniocentesis, said Dr. William Scott, of the AMA's Council on Scientific Affairs and associate professor of obstetrics at the University of Arizona in Tucson.

"What we're saying is this appears very promising, but opinion is divided and not all the facts are in," Scott said. "Whether a woman should undergo this procedure or not would depend on a number of individual factors. You can't make a blanket recommendation."

With more older women becoming pregnant, doctors have begun to test amniotic fluid routinely to determine whether a fetus has genetic abnormalities such as trisomy 21, which causes Down's Syndrome. The detection of serious defects may result in a decision to abort the fetus.

In an attempt to learn of such defects earlier -- making an abortion, if necessary, far less complicated -- Scandinavian researchers in the late 1960s began using an ultrasound-guided catheter to remove a small piece of the chorion, the outermost membrane of the placenta. The procedure since has been refined and about 6,000 have been performed in the United States, and more than 21,000 internationally.

After reviewing the evidence to date, 31 experts on the AMA's Diagnostic and Therapeutic Technology Assessment (DATTA) panel split evenly on the issue of effectiveness, with 15 saying it had been established, 15 calling it investigational and one saying it could not be determined.

Only seven of the specialists, who reported their results in the Journal of the American Medical Association, said the safety of CVS had been established, with 22 calling it investigational, one calling it unacceptable and one indeterminate.

The panel was concerned because of the rate of spontaneous abortion after the procedure was high, 4 percent.