Kathy felt pretty confident that she was protected by her birth control pills. But she put on a lot of weight over the winter and spring, and by summer, extra weight wasn't all she was carrying.

I'm Dr. Caroline Cederquist, giving you The Skinny on Your Health.

New research shows that overweight women had a 60 percent higher risk of failure with their oral contraceptives than women of normal weight.

Women who weigh 155 pounds or more had a higher risk of pregnancy when on very low dose birth control pills. This is very important as the average woman in the US over age 20 is 152 pounds.

The researchers believe the higher failure rate may have something to do with how the women metabolize the oral contraceptives, as the hormones that make them effective are fat-soluble and may become too diffuse in a woman with more body fat.

Clinical trials for oral contraceptives typically don't include overweight women, but that's not the real world, so it's important for them to know that they are not protected at the level others may be.

And that's true even if, like Kathy, the weight gain was recent. It's just that sort of unexpected change that could leave a woman, well, expecting.

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Caroline J. Cederquist, M.D. is a board certified Family Physician and a board certified Bariatric Physicians (the medical specialty of weight management). Dr. Cederquist is the founder of Bistro M.D., a home diet delivery program that specializes in low calorie gourmet food that is delivered to your home or office. Bistro M.D. serves as culmination of Dr. Cederquist's expertise and experience in the world of medical weight loss.