One half of the 21 million U.S. women who use reversible contraception choose oral contraceptive pills (OCPs). OCPs appear to fail in practice in this group, resulting in more than 500,000 unintended pregnancies every year. Little research has been conducted on this topic, but high body weight has been associated with low serum levels of ethinyl estradiol and OCP failure. Holt and colleagues studied the relationship between body weight and OCP failure in 755 women enrolled in a health maintenance organization.

Patients were 18 to 39 years of age and enrolled as control subjects in a study of ovarian cysts and neoplasms. As part of this study, the women provided extensive data on several personal, lifestyle, and medical factors, including OCP use. Information on the type of OCP used was validated by pictures of specific brands. The women reported details of each conception, including how it was confirmed and what type of contraception (if any) was used at the time of conception. Height and weight at the time of each conception were also determined.

During 2,822 woman-years of OCP use, 106 (3.8 per 100 woman-years) pregnancies were confirmed. The statistical analysis adjusted for multiple failures by individual patients and the possible confounders of age, smoking, gravidity, parity, and menstrual irregularity. The analysis also included the effects of compliance, race, and type of OCP used. The average patient was followed for 12.5 years, and one half of the participants had two or more pregnancies during this time.

Women in the highest quartile of body weight (70.5 kg [155 lb] or more) had a significantly increased risk of OCP failure, with a relative risk of 1.6. The risk of failure was particularly elevated in women using very-low-dose OCPs.

The authors conclude that women weighing 70.5 kg or more had a 60 percent increased risk of OCP failure, and that this risk increased with use of lower doses of estrogen. These results correlate with information from other studies, including tests of a transdermal contraceptive patch. The effect could result because of differences in metabolism in obese women or because of decreased circulating levels of lipophilic contraceptive steroids. The authors caution physicians to consider body weight in recommending OCP use for obese women.