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A new study from the Fred Hutchinson Cancer Research Center reports that women on birth control pills who are categorized by their body-mass index as overweight or obese are 60 to 70 percent more likely to get pregnant than "normal" weight women who also take the pill.
One possible explanation is increased metabolism. "The more a person weighs, the higher their basal metabolic rate, which can shorten the duration of a medication's effectiveness," she said. Another possibility is that the heavier a person is, the more liver enzymes they have to clear medications from the body, causing a drop in circulating blood levels of the drug. A third theory is based on the fact that the active ingredients in oral contraceptives – the hormones estrogen and progesterone – are stored in body fat. "The more fat a person has, the more likely the drug is sequestered, or trapped, in the fat instead of circulating in the bloodstream," (epidemiologist Victoria) Holt said.Link