Objective: To estimate the effect of body mass index (BMI) and weight on risk of pregnancy while using oral contraceptives (OCs).
Methods: We conducted a case-control study of 248 health maintenance organization enrollees who became pregnant while using OCs between 1998 and 2001 and 533 age-matched enrollees who were nonpregnant OC users during the same period. Using logistic regression we calculated adjusted odds ratios (ORs) to estimate the risk of pregnancy according to BMI and weight quartile.
Results: Among all OC users, when compared with women having a BMI of 27.3 or less, the risk of pregnancy was nearly 60% higher in women with BMI greater than 27.3 (OR 1.58, 95% confidence interval [CI] 1.11-2.24) and over 70% higher in women with BMI greater than 32.2 (OR 1.72, 95% CI 1.04-2.82). Among consistent users (women who missed no pills in reference month), the risk of pregnancy was more than doubled in women with BMI greater than 27.3 (OR 2.17, 95% CI 1.38-3.41) or BMI greater than 32.2 (OR 2.22, 95% CI 1.18-4.20). When compared with women weighing 74.8 kg or less, among consistent OC users the risk of pregnancy was over 70% higher in women weighing more than 74.8 kg (OR 1.71, 95% CI 1.08-2.71) and nearly doubled in women weighing more than 86.2 kg (OR 1.95, 95% CI 1.06-3.67).
Conclusion: Our results suggest that being overweight may increase the risk of becoming pregnant while using OCs. If causal, this association translates to an additional 2-4 pregnancies per 100 woman-years of use among overweight women, for whom consideration of additional or effective alternative contraceptive methods may be warranted.