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LYON, France - An older woman's slim chances of getting pregnant could be made worse if embryos are screened for defects before being implanted into the womb, doctors said Wednesday.

Pregnancy and live birth rates were substantially lower among women whose embryos were screened compared with those whose were not, according to a study presented Wednesday at a Lyon meeting of the European Society of Human Reproduction and Embryology.

The findings, which were questioned by some experts, suggest genetic screening should not be a routine part of fertility treatment for women over 35.

"Based on our research, I would say skip the pre-genetic screening and stick with regular in-vitro fertilization," said Dr. Sebastiaan Mastenbroek from the Center for Reproductive Medicine at the University of Amsterdam.

In the study, also published Wednesday in the New England Journal of Medicine, Mastenbroek and colleagues were trying to determine the value of pre-genetic screening, a process that involves taking a single cell from a developing embryo to look for chromosomal defects that could lead to problems such as Down's syndrome. Doctors have generally thought selecting the most promising embryos will give older women a better chance of getting pregnant.

But some experts have expressed concern that fertility centers promote the genetic tests because they generate profit - with a single test costing up to $5,000.

"This is the first study that has tried to determine for patients, if they put their money down, whether they will take a baby home," said Dr. Peter Braude, professor of obstetrics and gynecology at King's college. The answer, Braude said, was not necessarily.

The study involved 408 women, aged 35-41 and undergoing three cycles of in-vitro fertilization. The Dutch researchers tested embryos from half of those women.

Roughly half were assigned to have their embryos screened. Those who received the screening had a substantially lower pregnancy rate than those who did not: 25 percent versus 37 percent. More than 60 percent of the embryos implanted into women who had them screened were abnormal, a clear indication of the difficulties older women face getting pregnant. And in this study, having their embryos screened did not help.

Mastenbroek and colleagues had several hypotheses to explain the discrepancy between women who received screening and those who did not.

"It could be that the biopsy of one cell needed for pre-genetic screening is more harmful to the ultimate development of the embryo than we had previously thought," Mastenbroek said.

They also suggested that the cell examined might not be representative of the entire embryo. Even if the biopsied cell appeared normal, the rest of the embryo might be abnormal, leading to a problematic pregnancy.

Other experts had reservations about the researchers' methodology.

"The way they performed this process tells us that they did not have a lot of expertise in this," said Dr. Anna Ferraretti, scientific director of the Italian Society for the Study of Reproductive Medicine.

Ferraretti and others said that sometimes the researchers took one cell to biopsy from a four-cell embryo, earlier than is standard procedure for pre-genetic testing.

Some practicing doctors said they were not surprised by Mastenbroek's results. "We have not seen that doing pre-genetic screening increases pregnancy rates," said Dr. Paul Devroey, of the Center for Reproductive Medicine in Brussels. "There is still a big question mark over whether the screening of embryos helps."