NEW YORK (Reuters Health) – Despite studies suggesting that birth control pills might not work as well in obese women, a new study suggests that they prevent pregnancy the same no matter what a woman weighs.

As long as a woman-heavy or thin–took the pill consistently, Dr. Carolyn L. Westhoff of Columbia University Medical Center in New York City and her colleagues found, it prevented her ovaries from producing eggs. Westhoff and her team report their findings in the August issue of Obstetrics & Gynecology.

Studies have found that obese women are 30 to 40 percent more likely to get pregnant while taking the pill than were normal-weight women taking it, perhaps because excess weight might blunt its effectiveness. Overall contraception failure rates are estimated between 2 and 8 percent per year.

In their study, Westhoff and her colleagues enrolled 226 normal-weight or obese women, randomly assigning them to take a lower or higher dose version of the pill. After three or four cycles of oral contraceptives, the researchers then had the women undergo multiple ultrasounds and blood tests to determine if their ovaries were making eggs.

The researchers tested women’s blood levels several times to determine if they were using the pill consistently; about 90 percent of the normal-weight women had hormone levels indicating consistent use, compared to 72 percent of the obese women.

Ninety-seven percent of the 150 consistent pill users did not ovulate, the researchers found, meaning they would not have gotten pregnant during that cycle. Whether or not a woman was obese didn’t affect her likelihood of ovulation; among the consistent pill users, three of the 96 normal-weight women ovulated, and one of the 54 obese women did.

Two ovulations occurred in women taking the higher-dose pill, and two occurred in women on the lower dose.

But more than a third of the 13 women who were inconsistent pill users ovulated, and two-thirds of the 18 non-users produced eggs.

The new findings, along with more recent clinical trials, should put to rest concerns that obese women are more likely to get pregnant while on the pill, Westhoff says. The issue has confounded doctors, she notes, because some companies have pushed physicians to prescribe higher dose formulations to their obese patients based on the idea that lower doses aren’t effective.

The problem with this approach, she explains, is that obese women are at increased risk of developing life-threatening blood clots in their veins. Taking the pill-especially at higher doses-further increases the risk these clots will occur.

But according to her findings, Westhoff says, obese women who take lower-dose pill formulations can be reassured that they will be effective in preventing pregnancy.

Westhoff has consulted for drug firms that make oral contraceptives, including Duramed, Schering-Plough, and Bayer.

The problem with previous studies, Westhoff told Reuters Health, was that women were asked to recall unintended pregnancies that had occurred in the past; the women also reported their own weights, rather than being weighed by the research team.

Dr. Victoria Holt of the University of Washington in Seattle, who led the studies finding different failure rates among obese and normal-weight research, said she still believes obese women are more at risk of becoming pregnant while on the pill.

She argues that Westhoff’s research suggests obese women are less likely to achieve adequate hormone levels while taking the pill, because just 72 percent had levels indicating consistent oral contraceptive use, compared to 90 percent of the normal-weight women.

“If indeed there is lower effectiveness among obese women, then this does not appear to be the mechanism through which it works,” Dr. James Trussell, the director of the Office of Population Research at Princeton University in New Jersey, told Reuters Health. Trussell studies contraceptive failure, but wasn’t involved in Westhoff’s research. “But nobody has any idea what the other mechanism would be.”

This November, the National Institutes of Health will convene a group of experts to investigate whether further research should be done on obesity and oral contraceptives, he noted, while the Food and Drug Administration is also planning a hearing to address evidence that a new emergency contraceptive pill is more likely to fail in obese women.

“It’s a particular concern in the United States because obesity is increasing so rapidly,” Trussell said. “I would say that the pill is still not going to be the front line, the top tier contraception for obese women.”

Methods with a lower risk of failure due to human error, like the IUD, hormonal implants, or vasectomy for the male partner, would likely be a better choice, he said.

“If a woman really doesn’t want to become pregnant I would suggest combining oral contraceptives with a barrier method,” Holt advised, such as condoms or a diaphragm.