Overweight and on the pill? You might want to try backup contraception. A new study has found that being overweight or obese might increase the risk of getting pregnant while using oral contraceptives, suggesting that obesity may be an increasingly important cause of unintended pregnancy among women who take birth control pills.

What the researchers wanted to know: Does being overweight reduce the effectiveness of oral contraception?

What they did: The researchers analyzed the medical records of 248 women who became pregnant and 533 women who did not get pregnant while using birth control pills between 1998 and 2001. They then interviewed these women and used a statistical computer program to calculate the likelihood that a woman would become pregnant, based on her weight and body mass index (BMI), a measure of obesity. All women were enrolled in Group Health Cooperative, a health maintenance organization in Washington State.

What they found: Among all the women taking birth control pills, the risk of pregnancy was nearly 60 percent higher in women with a BMI greater than 27.3 and more than 70 percent higher in women with a BMI greater than 32.2. A BMI over 25 is considered overweight, while 30 or greater is obese. Among women who missed no pills, the risk of pregnancy more than doubled for women with a BMI greater than 27.3 or 32.2. It is not clear why the pill may not work as well for overweight women.

What this study means to you: If you are an overweight woman taking birth control pills, you may want to consider using additional or alternative contraception methods, like condoms or diaphragms. Talk to your doctor. Earlier studies have suggested that the same may be true of women using other forms of hormonal contraception—the implant Norplant and the patch.

Caveats: The study may have missed pregnancies that were not reported to the HMO. The researchers also relied on participants to self-report if they took the birth control pills regularly or if they missed any pills. Also, BMI is just one piece of someone's health profile, and a high BMI does not necessarily mean that a person is overweight. It overestimates fatness in people who are muscular or athletic. In other words, a bodybuilder with a large muscle mass and a low body fat percentage may have the same BMI as a person who has more body fat.

Find out more: Calculate your body mass index.

Also see Planned Parenthood's FAQ Fact Sheet on Birth Control.

Read the article: Holt, V. L., Scholes, D., Wicklund, K. G., Cushing-Haugen, K. L., and Daling, J. R. "Body Mass Index, Weight, and Oral Contraceptive Failure Risk." Obstetrics & Gynecology. January 2005, Vol. 105, No. 1, pp. 46–52.

Abstract online: www.greenjournal.org