Body mass index and risk for oral contraceptive failure: a case-cohort study in South Carolina.

PURPOSE Studies have suggested that obesity is associated with an increased risk for oral contraceptive (OC) failure. We conducted a case-cohort study in South Carolina to examine the association between body mass index (BMI) and OC failure by using population-based data sources. METHODS Our cohort sample from the source population consists of 205 women who reported using OCs to prevent pregnancy on the 1999 Behavioral Risk Factor Surveillance System survey. The 153 women who reported using OCs at the time of conception on the 2000 Pregnancy Risk Assessment Monitoring System survey represent the case sample that arose from the source population. Logistic regression was used to obtain odds ratios (ORs) and 95% confidence intervals (CIs). RESULTS In unadjusted models with normal BMI (20 to 24.9 kg/m(2)) as the comparison, greater BMI was associated significantly with OC failure (overweight [25 to 29.9 kg/m(2)], OR = 2.54; 95% CI, 1.18-5.50; and obese [> or =30 kg/m(2)], OR = 2.82; 95% CI, 1.05-7.58). After adjustment for education, income, and race/ethnicity, associations were attenuated and no longer statistically significant. CONCLUSIONS In this heterogeneous population, we found a suggestion that overweight and obese women may be at increased risk for OC failure. However, long-term prospective studies are needed to study this association in diverse populations.