The overweight women have a 60 to 70 percent higher risk of getting pregnant …

Now it turns out excess weight may be to blame when the &to=http://english.pravda.ru/fun/2003/01/23/42463.html' target=_blank>pill fails to prevent pregnancy.

Holt, an epidemiologist at the Fred Hutchinson Cancer Research Center analyzed data on nearly 250 Group Health Cooperative patients who became pregnant while on the pill and compared them to more than 500 pill users who didn't get &to=http://english.pravda.ru/science/19/94/377/12796_pregnancy.html' target=_blank>pregnant.

Pill users with a BMI over 32.2 had a 70 percent higher risk of pregnancy than women at a healthy weight.

The study didn't address the biology behind the link, but the researchers propose possible theories. First, the more a person weighs, the higher the metabolic rate, which may shorten the pill's activity. Second, heavier people have more liver enzymes, which means the pill may be cleared faster from the system.