Oral contraceptives seem to work equally well in women who are overweight and those who are not, according to a new study from Europe.

But the author of another recent report, which showed differences in the way obese women’s bodies respond to the pill, says the new findings aren’t the final word on the subject. “I don’t think the new study completely shuts the door on the question,” Dr. Alison Edelman of Oregon Health & Science University in Portland noted in an email to Reuters Health.

Research on the effects of weight and oral contraceptives has had mixed results, note the authors of the new study, which is published in the American Journal of Obstetrics and Gynecology.

To investigate, they analyzed data from a five-year study of 59,510 oral contraceptive users that had been designed to assess risks of different types of oral contraceptives.

During the study, 545 unplanned pregnancies occurred.

According to Dr. Jurgen C. Dinger of the ZEG-Centre for Epidemiology and Health Research in Berlin and colleagues, there was no evidence that a woman’s body mass index (BMI) influenced the likelihood that she would become pregnant while on most types of oral contraceptive.

However, the subset of contraceptives containing chlormadinone acetate (CMA) were less effective in heavy women with BMIs of 30 or greater or who weighed at least 75 kilograms (165 pounds). But according to the researchers, CMA-containing contraceptives’ effectiveness was still “acceptable.”

The low failure rate they found suggests that women in the study were using the pill correctly, the researchers note, so it’s not clear whether the findings would apply in populations where compliance wasn’t as good, or in groups of women where overweight and obesity were more common. “Nevertheless, these potential limitations should not detract from the strengths of the current analysis,” they conclude.

In their study, Edelman and her team looked at the pharmacokinetics of the pill, or the way the drug is processed in the body. They found that, among obese women, it took 10 days for hormones to reach levels required for suppressing ovulation, on average, compared to only five days for normal-weight women.

This could provide a window of opportunity for the ovary to release an egg, making pregnancy a possibility, these investigators say, although Edelman points out that the findings don’t prove that these women would have actually gotten pregnant.

Edelman noted that in her study, all 20 participants were known to be ovulating, while this wasn’t the case in the European study, which could have affected the results.

“Rates of obesity aren’t as high in Europe, and compliance rates have been shown to be higher in Europe as well, so it isn’t completely generalizable to the US population,” Edelman added.

SOURCE: American Journal of Obstetrics & Gynecology, October 2009.