Overweight and obese women who take oral contraceptives are up to 70 percent more likely to get pregnant than women of lower weight, say new findings from the Fred Hutchinson Cancer Research Center that appear in Obstetrics & Gynecology.
Study leader Victoria Holt said the study was the largest of its kind to examine the link between body-mass index and oral-contraceptive failure. "The results of our study represent yet another reason why obesity is a health hazard," Holt said. "Overweight and obese women have a significantly higher risk of getting pregnant while on the pill than women of normal weight, and this translates into a substantial number of unplanned pregnancies." Out of 100 women taking oral contraceptives for a year, the study suggests that an additional 2 - 4 women will get pregnant due to being overweight or obese. "This higher risk of pregnancy also translates into a higher number of obesity-related complications of pregnancy, which range from gestational diabetes and high blood pressure to Cesarean delivery," Holt said.
While no one knows for sure why overweight and obese women experience a higher degree of oral-contraceptive failure than their thinner counterparts, several biological mechanisms could account for the effect. 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," Holt 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," Holt said.
Another factor that may account for obesity-related pill failure is that today's oral contraceptives contain relatively low hormone levels compared to those first introduced four decades ago. "Hormone levels have decreased fivefold in an effort to reduce unwanted side effects ranging from weight gain to risk of blood clots and stroke," Holt said. "Today's pill dose is high enough for most women but may not be adequate for all."
Holt said that overweight women should not neccesarily demand a higher-dose pill to counter this. "Overweight women are more likely to have cardiovascular-disease risk factors than normal-weight women, and to increase the dose of contraceptive hormones might increase cardiovascular risks even more," she said. Instead, Holt recommends that overweight women who have completed childbearing consider a permanent form of birth control such as sterilization, and that overweight women who have not yet completed childbearing consider using a backup form of birth control while on the pill, such as condoms.