Oregon Health & Science University researchers say birth control pills may not boost women’s weight, a finding contrary to the commonly held belief that oral contraceptives cause weight gain.

The study will appear in the journal Human Reproduction next month.

Researchers conducted their research on a group of rhesus macaque monkeys, whose productive system is nearly identical to humans, for almost a year and found the birth control pills did not affect those monkeys’ food intake, activity or lean muscle mass.

"Issues surrounding weight are hard to study in humans, and the research thus far has been insufficient to demonstrate whether or not oral contraceptives cause weight gain or loss. But this is an extremely important question as concern about weight gain is one of the main reasons why women may avoid or discontinue birth control, which in turn places them at greater risk for an unplanned pregnancy," said Alison Edelman, lead author of the study.

Monkeys were equally divided into two groups: one was obese group and the other was normal group.

During the eight-month test, they were fed doses of oral contraceptives, which was the same intake as humans do.

The researchers tracked weight, food intake, body fat, activity levels and lean muscle mass, and found the normal group remained weight stable whereas the obese group lost 8.5 percent weight and 12 percent body fat.

"This study suggests that worries about weight gain with pill use appear to be based more on fiction than on fact," said Judy Cameron, Ph.D., senior author of the study.

Birth control pills are the most common way of contraception in the US and are used by over 100 million women worldwide. If taken consistently and correctly, they are more than 99 percent effective at preventing pregnancy.

"Early in 2003, researchers from Family Health International conducted a systemic research on 39 clinical trials of women using birth control pills, but found that there was no scientific proof that the pills caused an increase in weight. A widely held myth suggests that oral contraceptives cause weight gain, but the answer as best we can tell is they do not," said coauthor Dr. David Grimes of Family Health International.

According to Centers for Disease Control and Prevention, women who have used oral contraceptives are approximately half as likely to develop ovarian and endometrial cancer as women who have never used them and contraceptive use does not appear to increase a woman's risk of breast cancer.

“We realize that research in nonhuman primates cannot entirely dismiss the connection between contraceptives and weight gain in humans, but it strongly suggests that women should not be as worried as they previously were.", added Grimes.

Stephen Lau and editing by Aimee Keenan-Greene