Association between efficacy and body weight or body mass index for two low-dose oral contraceptives

BackgroundThis analysis investigated the association of oral contraceptive efficacy with body weight and body mass index (BMI) for hypothesis-generating purposes.Study DesignData were from a randomized, parallel-group trial of 180/215/250 mcg of norgestimate (NGM)/25 mcg of ethinyl estradiol (EE) (given to 1671 women) and 1 mg of norethindrone acetate (NETA)/20 mcg of EE (given to 1139 women). Pregnancies were evaluated across BMI deciles and by BMI and body weight dichotomies. A Pearl index was calculated for each treatment group. The relative risk (RR) of pregnancy was calculated with a Cox proportional hazards model.ResultsThe Pearl index for women who received NGM/EE was 2.36 [95% confidence interval (CI)=1.33–3.40]; for those who received NETA/EE, the Pearl index was 3.29 (95% CI=1.81–4.77). Consistent, weak positive associations between weight and pregnancy risk were found. Overall, for women with a BMI ≥25 kg/m2 (compared with women with a BMI <25 kg/m2), the RR of pregnancy was 1.84 (95% CI=0.98–3.45); that for women who received NGM/EE was 1.39 (95% CI=0.57–3.40), whereas that for women who received NETA/EE was 2.49 (95% CI=1.01–6.13). For women with a body weight ≥70 kg (compared with women with a body weight <70 kg), the RR was 1.25 (95% CI=0.63–2.46); that for women who received NGM/EE was 1.41 (95% CI=0.56–3.54), whereas that for women who received NETA/EE was 1.12 (95% CI=0.40–3.12).ConclusionWomen in the higher body weight or BMI category showed a small increase in the risk of pregnancy with these oral contraceptives, but this increase was not statistically significant overall or for either formulation studied.