Conflicting data have been reported for the contraceptive failure rate among overweight and obese females who use combined hormonal contraception (oral contraceptive pill [OCP] and the contraceptive patch and ring). Few studies have examined clinical outcomes in females who were more than 130% of ideal body weight. Because the risk of hormonal contraceptive failure with increasing BMI is unclear, it is difficult to provide accurate contraceptive counseling to obese women on failure rates.
The primary aim of this study was to investigate the possible association of contraceptive failure in women using the OCP, patch, or vaginal ring with increasing body mass index (BMI). Data were obtained from follow-up of participants enrolled in the Contraceptive CHOICE Project, which was a prospective cohort study involving reproductive-age females in the St Louis area. Each participant was provided the reversible contraceptive method of her choice at no cost and was followed up for 2 to 3 years. Failure rates (pregnancy) stratified by BMI were compared for users of the OCP, transdermal patch, and contraceptive vaginal ring.
At enrollment, 1523 of the 7486 participants who were available for analysis chose OCPs, patch, or ring. A total of 334 unintended pregnancies occurred during the study period; 128 of these were the result of OCP, patch, or ring failure. There was no difference in 3-year failure rates across BMI categories (3-year contraceptive failure range, 8.4%–11.0%; 95% confidence interval [CI], 6.1%–16.0%) (probability of contraceptive failure over time, P = 0.34). Significant risk factors for unintended pregnancy included increasing parity (hazard ratio, 3.06; 95% CI, 1.31–7.18) and history of a previous unintended pregnancy (hazard ratio, 2.82; 95% CI, 1.63–4.87). Body mass index was not a significant risk factor for unintended pregnancy.
These data indicate that overweight and obese females are not at increased risk for contraceptive failure when using the OCP, patch, or vaginal ring.