Effect of BMI and body weight on pregnancy rates with LNG as emergency contraception: analysis of four WHO HRP studies

Objective To estimate the effect of increased body weight and body mass index (BMI) on pregnancy rates with levonorgestrel (LNG) 1.5mg used as emergency contraception (EC). Methods The study reviewed data from 6873 women in four WHO-HRP randomized trials on EC conducted between 1993 and 2010. Participants took either 1.5mg of LNG as a single dose or in two doses 12h apart, up to 120h of unprotected intercourse. Contraceptive efficacy (pregnancy rates) at different weight and BMI categories was evaluated. Results Overall pregnancy rate was low at 1.2%. Pregnancy rates were also low in women weighing over 80kg (0.7%) and who were obese (BMI over 30kg/m2) (2.0%). The pooled analyses for pregnancy demonstrated that BMI over 30kg/m2 decreased efficacy significantly (odds ratio 8.27, 95% confidence interval = 2.70–25.37) when compared to women in lower BMI categories, mainly influenced by pregnancies in obese women from one study site. Sensitivity analyses excluding that site showed that obesity was no longer a risk factor; however, the other studies included too few obese women in the sample to exclude a substantial decrease in efficacy. Conclusions Pregnancy rates with use of LNG 1.5mg for EC were low at less than 3% across different weight and BMI categories. Pooled analyses showed an increase in pregnancy rates among obese women (BMI more than 30kg/m2) compared to women with normal BMI levels, influenced by pregnancies all coming from one study site. Implications Access to LNG as EC should still be promoted to women who need them, and not be restricted in any weight or BMI category, with additional attention for counselling and advice for obese women.