Heavy women often have a harder time getting pregnant than their slimmer peers, but new findings suggest weight may cease to matter when women take fertility drugs.

In a report in the journal Fertility and Sterility, scientists say that the “weight effect” is overcome by a higher dose of ovulation-stimulating drugs, which ensures obese women have a similar concentration of the hormones as those with a lower body mass index (BMI), a ratio of weight to height.

The report shows 25 percent of normal, 37 percent of overweight, and 36 percent of obese women who sought out treatments conceived with them.

These fertility rates are about equal, even though they look slightly inflated in larger women, said Dr. Irene Souter, lead author on the study. This is because the study included fewer heavy women, so just one more pregnancy means a big difference.

With the higher drug dose, “you bring the chances of conceiving close to that of a normal woman, and you’ve got a good chance of a live birth,” she said. “I wouldn’t say that obese women do better, I would say that they have a comparable outcome.”

A problem with requiring a bigger dose of fertility drug is that the costs go up, Souter said, with an obese woman paying, on average, roughly $200 more than the lower BMI categories. The total cost of this treatment, which is a combination of fertility drugs and artificial insemination, can range from several hundred dollars to around $1200.

There are also slight risks associated with the ovulation-stimulating injections, including enlarged ovaries, multiple births and pregnancy complications, according to the American Society for Reproductive Medicine.

Age is the biggest determinant of how easily a woman can get pregnant. Without fertility treatments, a woman in her 20s trying to get pregnant has about a 50 percent chance of succeeding per menstrual cycle. That number declines with age and drops significantly after 35.

The study looked at 477 women who had not had previous infertility treatments, and who started the treatments at the same time in their menstrual cycle. Underweight and extremely obese women were excluded, because these groups tend to have abnormal ovulation.

But the researchers did not compare their findings to similar women not on fertility drugs, said Dr. Calvin Greene, a fertility expert at the University of Calgary. For the study to be truly useful he said, these data are needed.

Doctors don’t know why heavier women have more trouble conceiving. But they do know that they tend to have increased problems in pregnancy, such as high blood pressure, diabetes, low birth weight, and delivering premature babies.

“The point is that it’s harder to achieve pregnancy, and you get higher risk pregnancies when you do achieve them,” Greene said.

Unfortunately, he said, more useful studies, such as seeing if losing weight will give obese women a better chance of getting pregnant, don’t exist. Still, both Souter and Greene say that it’s better to have a normal BMI before trying to have a baby.

“We just need to get a little lighter and a little healthier,” Greene said.

SOURCE: Fertility and Sterility, online December 31, 2010.