THURSDAY, Nov. 9 (HealthDay News) -- If you thought all the risks of cigarette smoking were already known, think again.

New research finds that heavy smoking impairs women's fertility by reducing the odds that a fetus will implant in the uterus.

Previously, experts had thought that heavy smoking reduced fertility because of its effect on the ovaries. The new finding suggests tobacco deals women a double blow.

"Tobacco consumption reduces your pregnancy probability, not only due to the already known ovarian effects but also due to impaired uterine receptiveness," said Dr. Sergio R. Soares, lead author of the study and director of the IVI Clinic in Lisbon, Portugal.

"This is the first study that shows the clinical impact of cigarette smoking on uterine receptiveness," added Soares, whose study is published in the Nov. 9 online edition of Human Reproduction.

The take-home message remains the same: "A healthy pregnancy starts with a healthy mother," said Dr. Jennifer Wu, an obstetrician/gynecologist at Lenox Hill Hospital in New York City. "Quit smoking before you become pregnant."

The effect of cigarette smoking on the ovaries has been known for a while, she said. "There's often ovulatory dysfunction in heavy smokers, and they tend to have menopause at an earlier stage," Wu noted.

The authors of the study looked retrospectively at 741 non-heavy smokers (under 10 cigarettes a day) and 44 heavy smokers (over 10 cigarettes a day). All of the women had received oocyte donations as part of in vitro fertilization (IVF) between January 2002 and June 2005.

None of the women's partners smoked and none of the oocyte donors were heavy smokers.

According to the study, the lighter smokers had a significantly higher pregnancy rate (52.2 percent) than the heavy smokers (34.1 percent).

The fact that the oocytes were donated means the problem lies with the uterus, not the ovaries, the researchers noted.

Previous research had also shown that light smoking had no significant effect on IVF cycles.

Oddly, heavy smokers had about double the rate of multiple pregnancies (60 percent) than non-heavy smokers (31 percent). Although this may be a glitch in the findings, it's also possible that different women respond differently to cigarette smoking, Soares said.

Soares and his team are now looking at the genetics behind the phenomenon.

"We are beginning a study of gene expression in the endometrium of heavy smoking oocyte recipients to see which might be the key molecules involved in the implantation process that are altered in these patients," he said.

More information

For more information on infertility, visit the March of Dimes.

SOURCES: Sergio R. Soares, M.D., Ph.D., director, IVI Clinic, Lisbon, Portugal; Jennifer Wu, M.D., obstetrician/gynecologist, Lenox Hill Hospital, New York City; Nov. 9, 2006, Human Reproduction online

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