Overweight and obese women who take oral contraceptives are 60 percent to 70 percent more likely to get pregnant while on the birth-control pill, respectively, than women of lower weight, according to new findings from Fred Hutchinson Cancer Research Center that will be published in the January issue of Obstetrics & Gynecology.
The study, led by epidemiologist Victoria Holt, Ph.D., M.P.H., a member of Fred Hutchinson's Public Health Sciences Division, is the largest case-control study of its kind to examine the link between body-mass index and oral-contraceptive failure. The research was conducted in collaboration with Delia Scholes, Ph.D., a senior investigator at the Center for Health Studies at Group Health Cooperative in Seattle.
"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." Among 100 women taking oral contraceptives for a year, Holt's study suggests that an additional two to four 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.
Body-mass index, or BMI, is a measure of body fat based on height and weight. The World Health Organization divides BMI into four categories for both men and women: underweight (18.5 or lower), normal (18.5 to 24.9), overweight (25 to 29.9) and obese (30 or greater). A BMI calculator is available on the National Institutes of Health Web site at http://nhlbisupport.com/bmi/bmicalc.htm
"We found little difference or variation in the risk of contraceptive failure among women who fell into the first three quartiles of body-mass index, but the risk jumped up dramatically among women in the fourth quartile," said Holt, also a professor of epidemiology at the University of Washington School of Public Health and Community Medicine.
Holt and colleagues found the association between extra pounds and pill failure first surfaced among overweight women whose body-mass index was 27.3 or higher, which is equivalent to a 5-foot, 4-inch woman who weighs 160 pounds or more. These women faced a 60 percent greater risk of getting pregnant while on the pill. Those considered obese, with a BMI of 32.2 or greater, faced a 70 percent greater risk.
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," 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," Holt said.