New York-Presbyterian Hospital/Columbia University Medical Center researchers, who conducted the study, said that the implications are important since pregnancy itself is riskier among women with obesity.
Dr. Carolyn Westhoff and her colleagues did not rely on women's recollections of how much they may have weighed at a time when the pill had failed and they became pregnant.
"We wanted to study what was actually happening in the ovaries of women and not depend on memory, which is notoriously faulty," Westhoff said.
In the study, after three or four months of using the oral contraceptives (time it usually takes for a woman's body to acclimate to the pill), the women had multiple ultrasounds and blood tests to determine if ovulation was being suppressed.
Of the 150 women who used the pill consistently, three of the 96 women with normal weight ovulated, as did one of the 54 women with obesity.
Also, when women were not taking the pill regularly, they ovulated with greater frequency.
"Our findings strengthen the message to patients that the pill will only work if it is taken every day. Weight does not seem to have an impact on suppression of ovulation, but consistency of pill-taking does," Westhoff said.
"Knowing that the lower dose works as well as the higher dose will allow physicians to not only help women with obesity avoid unwanted pregnancies, but also protect them from the possible health risks associated with higher doses," she added.
The study is published in the August issue of the journal Obstetrics and Gynaecology.