Obesity and the Pill: Do They Mix?

Peter Kovacs, MD, PhD


May 10, 2013


Despite the availability of highly effective contraception, about one half of all pregnancies are unplanned, and about one half of these gestations are terminated.[2] Contraception fails when it is not used perfectly, but other risk factors might affect the effectiveness of contraception. High BMI affects absorption, metabolism, distribution, protein binding, and clearance of various drugs. It is important to study the contraceptive efficacy of available hormonal methods in women at the extremes of body weight.

Obesity also influences a woman's chance of conceiving. Women who fall outside the normal BMI range have a lower chance of conceiving, and the higher the BMI, the lower the chances of becoming pregnant.

Obese women are more likely to have other underlying medical problems (eg, glucose intolerance, diabetes, cardiovascular disease, increased risk for thromboembolism) and must be careful with their choice of contraception. For the same reasons, it may be even less desirable for these women to conceive, because pregnancy, delivery, or termination would carry excess risks. They are also more likely to take other medications that could affect the activity of exogenous hormones. Obese women might also benefit from hormonal contraception, beyond the contraceptive effects. The otherwise increased endometrial risk associated with obesity is reduced with hormonal contraception.

The question of which method would best suit obese women, with respect to contraceptive efficacy as well as other medical considerations, must be answered. This study provides reassuring evidence about the contraceptive efficacy of hormonal methods even in women in the highest BMI categories. However, such women also tend to be older, and advanced age is associated with infertility. This may explain, to some degree, why hormonal methods of contraception were not affected by BMI.

Contraceptive efficacy, the noncontraceptive benefits, and the potential harms associated with combined hormonal methods should be further studied. Obese patients, however, can be reassured that they do not need to worry about an increase in contraceptive failure when they use this method properly.