Scientists have yet to determine the exact biological mechanism at work, but they suspect several factors may contribute to the higher rate of failure. For one thing, the more you weigh, the higher your metabolism—which may shorten the duration of a medication’s effectiveness. This may be unwelcome news for those who like to blame a sluggish metabolism—as opposed to overeating or inactivity—for excess pounds, but it’s simple physics: It takes more muscle to lug around more fat, and more muscle means a higher metabolic rate. Another possible explanation is that fat-soluble birth control hormones (estrogen and progesterone) may get “trapped” in excess fat and not be properly processed in the body.
In addition, new research suggests that entering pregnancy with extra pounds increases your chances of miscarriage by 67 percent. British researchers analyzed 16 studies with a combined 16,000 female subjects and found that those with a BMI of 25 or higher were dramatically more likely to suffer miscarriage within the first five months of pregnancy. In real terms, that means a 5-foot-4-inch woman weighing 145 pounds (prior to pregnancy) would be two-thirds more likely to miscarry than a woman who weighed 15 pounds less.
Overweight women, according to University of Pittsburgh researchers, also are twice as likely—and obese women three times as likely—to develop preeclampsia as their normal-weight peers. And the risk rises sharply even with small increases in prepregnancy body fat.
Other reproductive risks posed by excess prepregnancy weight include infertility, gestational diabetes, congenital malformations and an increased likelihood of the child becoming overweight later in life.