Many women stop using the pill because they think it's causing them to pack on the pounds. But new research found that simply isn't true.

A comprehensive review of previous studies published in the current issue of the Cochrane Library found no evidence that birth control pills cause weight gain.

"There's evidence that women may switch methods of birth control or stop using the pill because they think it contributes to weight gain," said Laureen Lopez, a review co-author from Family Health International. "The good news is that this review found no evidence to support a causal association between combination contraceptives and weight gain."

Of the 44 studies that were reviewed, only three compared a hormonal contraceptive to an inactive placebo. None of them showed a significant increase in weight in women using the contraceptive compared to those using the placebo.

Of the 41 head-to-head studies that compared different hormonal contraceptives or different doses or regimens, most comparisons did not show substantial differences in weight.

"Over time, many people do put on weight, and some women's perception about weight gain may be that it's because of the pill," Lopez said in a news release.

In a national survey of American women published in 2000, half believed that oral contraceptives cause weight gain, and one in five women claimed that fear of gaining weight was a reason they would not start or would discontinue the pill.

Myths about birth control-related weight gain are shared by many doctors and health educators, said Dr. Carolyn Westhoff, a professor of obstetrics and gynecology at Columbia University.

"Overwhelmingly, doctors have the same incorrect myths as the general population, and sometimes the counseling that patients receive by health educators incorporates and deepens these myths," she said.

• Review Abstract