Birth Control Pills Effective for Women of All Sizes

Aug. 18, 2010 -- As long as they use oral contraceptives consistently, obese women have the same risk of pregnancy as those of normal weight, researchers have found.

In a randomized, controlled trial, prevention of ovulation was "substantial and comparable" among consistent users of birth control pills, regardless of their weight, according to Dr. Carolyn Westhoff of the Columbia University College of Physicians and Surgeons and colleagues.

The finding should lay to rest fears that oral contraceptive pills are less effective for obese women, Westhoff and colleagues argued in the August issue of Obstetrics & Gynecology.

Previously reported differences in unintended pregnancy, they argued, "must be attributable to other factors," such as less consistent use.

Indeed, an unexpected finding of the study was that the rate of consistent use among the obese participants was only 72 percent, compared with 90.6 percent among those of normal weight, the researchers reported.

"As a physician, I am relieved by the results of this study," Westhoff said in a statement. "When I prescribe oral contraceptives to my patients with obesity, I can feel confident that I am giving them something that will work."

The researchers noted that clinical trials of contraceptive pills typically include women of near-normal weight, but more than 60% of reproductive-age women in the U.S. are now either overweight or obese. If the pills are less effective in the obese, they wrote, clinical trial data will overestimate the effectiveness of the medications.

To clarify the issue, Westhoff and colleagues enrolled 226 women, including 128 of normal weight (defined as having a body mass index from 19 through 24.9) and 98 who were obese (defined as a body mass index of 30 or higher).