Being overweight dramatically increases the chances of women on the pill getting pregnant.
A woman who is overweight or obese is 60 percent to 70 percent more likely to conceive while taking an oral contraceptive than one who is slim, researchers found.
The reason for the higher pregnancy risk is not fully understood, but may be linked to the way contraceptive drugs are processed in people with excess body fat.
Scientists in America compared 248 women who became pregnant on the pill with an age-matched comparison group of 533 non-pregnant women using the same form of birth control.
Research leader Dr. Victoria Holt, from the Fred Hutchinson Cancer Research Center in Seattle, said: “The results of our study represent yet another reason why obesity is a health hazard. 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 Caesarean delivery.”