Obesity is prevalent in reproductive age women and body mass index (BMI) can affect serum levels of contraceptive hormones. We sought to determine if BMI decreased etonogestrel levels in contraceptive implant users, especially as implant use reached three years and beyond.
We enrolled 10 normal weight (BMI<25 kg/m2), 19 overweight (BMI ≥25 and <30 kg/m2) and 23 obese (BMI ≥30 kg/m2) women from the community using the etonogestrel implant for contraception for more than one year. We recorded demographic information and BMI and collected serum to measure etonogestrel using liquid chromatography mass spectrometry. The average age of participants was 28 ±6 years and 87% were Hispanic. The median BMI among obese women was 35 kg/m2, range 31-56. Thirty-two percent used the implant between 1-2 years, 36% for 2-3 years and 32% for 3-4.4 years. Median etonogestrel levels in the normal weight (216 pg/mL; IQR 214), overweight (288 pg/mL, IQR 145), and obese groups (225 pg/mL, IQR 111) were comparable (p=0.2). There was a weak relationship between time since insertion and etonogestrel level (p=0.1). After controlling for time since insertion, there was no association between BMI and etonogestrel level (p=0.7). All participant etonogestrel levels were above the threshold required to inhibit ovulation (90 pg/mL); the lowest level recorded was 117 pg/mL. For women of any BMI, etonogestrel levels remain above the levels required to inhibit ovulation, even at three years of use. These results support published high contraceptive efficacy rates for contraceptive implant users across BMI categories.