The effect of BMI on unintended pregnancy rates among users of combined oral contraceptives

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Objectives: We aimed to determine if the Pearl Index of combined oral contraceptives differs by BMI. Methods: We conducted a meta-analysis of five prospective, observational cohort studies with primary endpoints of venous thromboembolism (VTE) among women using combined oral contraceptives. Studies were conducted between 2007 and 2016 using a similar methodology. In sum, 246,209 women and 382,789 women–years were included. Women were followed for 3–5 years. The inclusion criterion was prescription of a new combined oral contraceptive. Studies were conducted across Europe and the United States. Results were analyzed within four age groups: younger than 25, 25–29, 30–39 and 40 or older. BMI was defined dichotomously as less than 35 kg/m2 and 35 kg/m2 or higher (US studies) and less than 30 kg/m2 and 30 kg/m2 or higher (Europe). The Pearl Index was calculated within each age and BMI category stratified by region. Significance of factors was tested in a stratified Cox regression model; age and BMI were included as continuous variables. Results: In the United States, the Pearl Index ranged from 0.15 (age 40+, BMI<35) to 4.12 (age <25, BMI ≥35) with higher values observed among women with BMI 35 kg/m2 or higher within each age group. Significance (p<.0001) was obtained for both factors when simultaneously included in a Cox regression model. In the European sample, the Pearl Index ranged from 0.06 (age 40+, BMI<30) to 0.80 (age <25, BMI ≥30). Cox regression showed independent effects of age (p<.0001) and BMI (p=.0003) on the occurrence of an unintended pregnancy. Conclusions: BMI has a significant effect on the Pearl Index of combined oral contraceptives. Increasing BMI decreases the efficacy of combined oral contraceptives in Europe and the United States.