The Effect of Body Mass Index on Etonogestrel Hormone Levels in Women Using the Single-rod Contraceptive Implant

Two-thirds of reproductive-age women in the United States are either overweight or obese. Body composition may affect contraceptive hormone metabolism and possibly efficacy. Phase III studies to date included few women weighing more than 70 kilograms (134 of 923, 14.5%); there were no reported pregnancies in that group. Data about pregnancy rates for women in higher body mass index (BMI) categories using the etonogestrel implant exists, but we know little about how weight affects the serum etonogestrel levels. Progestin-only methods, such as the etonogestrel implant, may be preferred over combined methods including estrogens because obesity increases thrombosis risk. The investigators propose a prospective study to compare serum etonogestrel levels in the second and third years of implant use between women across body mass index categories. Based on previous studies, the investigators expect a difference in etonogestrel levels based on body weight; however, the investigators hypothesize that etonogestrel levels will remain above the threshold for ovulation suppression through three years of implant use for women across BMI categories. The investigators will measure the serum levels of etonogestrel in normal weight (BMI \< 25kg/m2), overweight (BMI ≥ 25kg/m2 and \< 30kg/m2), and obese women (BMI ≥ 30kg/m2) using the single-rod contraceptive implant for at least twelve months. Their primary objective is to compare the three groups and to evaluate whether etonogestrel levels fall below the critical level for contraceptive efficacy (90 pg/mL) (Díaz 1991) in any group.