NEW YORK (Reuters Health) - The lining of the uterus or "endometrium" appears to play a small but significant role in reducing fertility among women who are overweight, Spanish researchers report.

The findings, they say, show that overweight and obese women undergoing infertility treatment with donor eggs should try to lose weight before becoming pregnant, which will give them the best chance of a good outcome.

Obese women are known to have more difficulty becoming pregnant and also are more likely to miscarry. Excess weight could exert its effects either by acting on the ovaries, which produce eggs, or on the endometrium, where the fertilized egg implants.

To investigate the role of the endometrium, Dr. Jose Bellver and colleagues from the University of Valencia looked at 2,656 women undergoing infertility treatment with donor eggs, all provided by non-obese women.

As body mass index (BMI) increased, the researchers found, pregnancy success rates declined. Among women with BMIs below 25, meaning they were of normal weight or underweight, the percentage of pregnancies that lasted beyond 20 weeks per cycle of treatment was 45.5 percent, compared to 38.3 percent for women with BMIs of 25 or greater.

Rates of miscarriage and ectopic pregnancy, meaning the fertilized egg had implanted outside the uterus, also rose in tandem with BMI.

In a report of the study in the journal Fertility and Sterility, Bellver and colleagues conclude that excess weight exerts an "extraovarian detrimental effect, and that its correction could improve the reproductive outcome in overweight and obese patients."

"The role of the endometrium or its environment seems to be small, but should be taken into account," they add.

SOURCE: Fertility and Sterility, August 2007.