"The results of our study represent yet another reason why obesity is a health hazard," said Victoria Holt, an epidemiologist with the Fred Hutchinson Cancer Research Center.

The higher pregnancy risk also means a more obesity-related complications, "which range from gestational diabetes and high blood pressure to Cesarean delivery," she said.

Holt and colleagues compared the weight and body-mass index of 248 women who became pregnant while on the pill to 533 pill users who were not pregnant to reach the conclusion that overweight or obese women are 60 to 70 percent more likely to get pregnant.

Holt said it was unknown why the pill failed more often in overweight women, but it could be because today's birth control pills have less hormone than earlier versions. She recommended overweight women consider other birth-control strategies.

The research involved Group Health Cooperative, a Seattle HMO from which the study pulled participants. Results will be published in the January issue of Obstetrics & Gynecology.