On the Pill? Extra Weight Ups Pregnancy Risk

What's Going On?

Excess body fat affects how well the body uses birth control hormones, Holt explains.

"Higher body weight affects metabolism, which may cause birth control hormones to be 'used up' more quickly," she tells WebMD. The hormones therefore offer less protection, since they aren't circulating in the bloodstream very long. Also, overweight people may metabolize hormones more quickly. That could be another reason why birth control pills aren't as effective in overweight and obese women, she explains.

Another factor: "Hormones in birth control pills are fat soluble, which means the hormones can be stored in fat," she says. "A woman with more fat storage may have more hormones trapped in that fat and not circulating in the bloodstream."

Given today's low-dose birth control pills, "it's reasonable to believe that it's not adequate for some women now," Holt adds.

What Should You Do?

Getting a higher-dose birth control pill is not the answer, says Holt.

"Overweight women have heart disease risk factors, and higher-dose birth control pills will make that risk greater," she tells WebMD. "Talk to your health care provider, and depending on where you are in your reproductive life, you may want to consider different options ... vasectomy, or tubal sterilization, or an IUD."

For women who have not completed childbearing, the pill is still more effective than other temporary birth control methods, says Holt. "But they might want to add another method like condoms, if it's important to them to not have a pregnancy."

Do women who lose weight have a lower risk of pregnancy? "We haven't looked at that," Holt tells WebMD, "but based on the mechanisms we're looking at, it makes sense."

SOURCES: Holt, V. Obstetrics & Gynecology, January 2005; vol 105. Victoria L. Holt, PhD, MD, epidemiologist, Fred Hutchinson Cancer Research Center and the University of Washington, Seattle. Paul Norris, MD, director of reproductive health, University of Miami School of Medicine.