Effect of body weight and BMI on the efficacy of levonorgestrel emergency contraception

Abstract Objectives To further evaluate the effect of weight and body mass index (BMI) on the efficacy of levonorgestrel emergency contraception. Methods Data from two large, multicenter, randomized controlled trials designed to assess emergency contraceptive efficacy were pooled to evaluate the effect of weight and BMI on pregnancy rates among women who received levonorgestrel. Descriptive methods (comparison of means and distributions according to pregnancy status and pregnancy rates across weight and BMI categories) as well as cubic spline modeling were used to describe the relationship between pregnancy risk and weight/BMI. Results The analysis population comprised 1731 women, among whom 38 pregnancies were reported. Women for whom levonorgestrel was not effective in preventing pregnancy had a significantly higher mean body weight and BMI than women who did not become pregnant (76.7 vs. 66.4kg, p 2 , p 85kg groups, respectively. Statistical modeling demonstrated a steep increase in pregnancy risk starting from a weight near 70–75kg to reach a risk of pregnancy of 6% or greater around 80kg. Similar results were obtained for statistical modeling of BMI as well as when the two studies were analyzed individually. Conclusions All analyses showed a significant drop in the efficacy of levonorgestrel emergency contraception with increasing body weight, with pregnancy risk in the higher weight categories similar to expected rates in the absence of contraception. Like body weight, increasing BMI was highly correlated with increased pregnancy risk.