We've known for awhile that birth control pills might be less effective for heavy women. A study reported in 2005 by the American College of Obstetricians and Gynecologists found that overweight women had a 60 percent greater and obese women a 70 percent greater chance of unintentional pregnancy while on the pill than women of normal weight.  These percentages applied to women who diligently remembered to take every pill, every day, precisely on schedule.  Those who occasionally forgot a pill were at an even greater risk of getting pregnant than forgetful women of normal weight.

Now, new studies done at Oregon Health and Science University (OHSU) confirm these earlier studies and say birth control pills work differently and not as well in obese women. 

Researchers aren't absolutely certain why overweight women are less protected by oral contraceptives than thinner women but they're researching several theories. One theory is that since the hormones in oral contraceptives are fat-soluble they may dissolve and deposit in overweight women's fat stores instead of circulating in the blood stream. 

OHSU's most current research says it took about 10 days of taking the pill for hormone blood levels to reach the optimum steady-state concentration required to suppress ovulation, compared to about 5 days for normal-weight women. This may provide a monthly "window of opportunity" for pregnancy to occur if ovulation isn't suppressed.

Older theories are that overweight women's metabolism may be different than normal-weight women and may change how hormones are absorbed.  Increased liver enzymes may affect how oral contraceptives are digested.  These factors may alter the amount of hormones that reach the reproductive system and prevent conception. 

Experts agree that more research is needed on overweight women and oral contraceptives before specific recommendations can be made.  Until then, heavy women should talk to their doctor about the safest, most effective methods of birth control for their bodytype and whether back-up contraception is needed.