No link between obesity, birth control failure

By Amy Norton

NEW YORK (Reuters Health) - While some research has suggested that obesity reduces the effectiveness of birth control pills, a new study indicates this may not be the case.

Researchers found that among 1,300 U.S. women who had been using oral contraceptives for several years, obese women were no more likely to say their birth control had failed them.

In all, about 10 percent had an unplanned pregnancy during the study period. Obese women did appear to be at higher risk, but when the researchers looked at other factors -- including age and race -- these seemed to largely explain the obesity finding.

"This population-based study found no association between obesity and oral contraceptive failure," the researchers report in the American Journal of Epidemiology.

Still, the findings are not the final word, according to lead researcher Dr. Larissa R. Brunner Huber.

"At this point in time," she told Reuters Health, "we aren't really certain if obesity increases a woman's risk of becoming pregnant while using birth control pills. There has not been a lot of research in this area."

What overweight women should not do, Brunner Huber said, is switch from birth control pills to a less reliable form of contraception. She added that all women, regardless of their weight, should be sure that they are using their birth control pills correctly.

The researchers based their findings on responses from 1,301 women who were interviewed in a national health survey in 2002. All had been using birth control pills since at least 1999.

Between 1999 and the time of the interviews, 142 women had an unintended pregnancy. The odds for unintended pregnancy were higher among women who were 25 years old or younger compared with older women; the same was true of African-American women versus white women.

When factors such as these were weighed, obesity, per se, was no longer linked to the risk of birth-control failure.

According to Brunner Huber, it's not clear why obesity would increase the chances of birth-control failure, as some studies have suggested. However, she explained, birth control pills contain hormones -- estrogen and progesterone -- that prevent ovulation, and body fat influences a woman's levels of these hormones.

"As a result of these changes," she said, "an obese woman taking birth control pills may not have the appropriate hormone levels needed to stop ovulation."

More studies are needed, however, to show whether this is the case, the researchers say.

SOURCE: American Journal of Epidemiology, December 1, 2007.