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Pill works less effectively for obese women
Updated: 2004-12-31 11:03

Overweight and obese women are 60 to 70 per cent more likely to get pregnant while taking an oral contraceptive than the women of normal weight, a new study shows.

Researchers from the US Fred Hutchinson's Public Health Sciences Division compared the weight and body mass indexes of 248 women who became pregnant while on the pill to that of 533 women who did not become pregnant while on the pill.

The result shows that women with a BMI of 27.3 or higher were 60-percent more likely to get pregnant while on the pill.

BMI of 27.3 is equivalent to a five-foot, four-inch woman who weighs 160 pounds or more. Women who were considered obese, with a BMI of 32.2 or higher, had a 70 percent greater risk.

Researchers suspect metabolism may be to blame, saying the more a person weighs, the higher their basal metabolic rate, which can shorten the duration of a medication's effectiveness.

Another explanation is heavier people have more liver enzymes that clear medications from the body.

Victoria Holt, Ph.D., lead author of the study, says, ¡°The more fat a person has, the more likely the drug is sequestered, or trapped, in the fat, instead of circulating in the bloodstream."

Holt makes recommendations that overweight women who have completed childbearing should consider a permanent form of birth control such as sterilization, and overweight women who have not yet completed childbearing should use a backup form of birth control such as condoms while on the pill.

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