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).
Of those randomized, 181 completed the trial, and 150 were found to be consistent users, based on measurements of hormones in the bloodstream, the researchers reported.
Westhoff and colleagues found:
Consistent users had substantial suppression of follicular development -- an indication of the effectiveness of the birth control pills.
Obesity and follicular development were not related.
Among the consistent users, 2.7 percent ovulated during the study, including three of the 96 normal-weight women and one of the 54 obese participants.
Inconsistent use or non-use was significantly associated with more ovulation.
Normal-weight and obese participants had similar follicular development.
The findings "strengthen the message to patients that the pill will only work if it is taken every day," Westhoff said. "Weight does not seem to have an impact on suppression of ovulation, but consistency of pill-taking does."