Objective: To study the mechanism of action of Implanon, a single-rod contraceptive implant containing etonogestrel, in healthy women during 3 years.
Design: Prospective, randomized comparison with Norplant. The study was conducted for 2 years in Sweden but was extended to 3 years in Finland.
Setting: Two outpatient clinics.
Patient(s): Thirty-two healthy women who were between 18 and 40 years of age with normal ovulatory cycles. Seven women receiving Implanon and three receiving Norplant participated in the third year.
Intervention(s): On or between days 1 and 5 of a spontaneous menstrual cycle, the subjects received either the etonogestrel-containing implant (Implanon) or the levonorgestrel-containing implant (Norplant).
Main outcome measure(s): Ultrasonography was performed and/or progesterone concentrations were determined to confirm ovulation in a control cycle. Follicular development, endometrial thickness, and serum concentrations of 17beta-estradiol and progesterone were assessed twice per week during 4-week periods at regular intervals and after implant removal for 6 weeks to monitor return of ovulation. Times required to remove the respective implants were evaluated, as were possible complications.
Result(s): Seven women who received Implanon and three who received Norplant completed 3 years of study. There were no pregnancies. Ovulation was observed for the first time with Norplant after 18 months. The first ovulation with Implanon was observed after 30 months. Mean endometrial thickness was <4 mm during treatment with Implanon from month 12 onward. The mean (+/-SD) time to remove Implanon was 5.9 +/- 3.4 minutes. The mean (+/-SD) time to remove Norplant was 17.9 +/- 9.9 minutes. Ovulation resumed promptly after the use of either implant.
Conclusion(s): Results from this study provide convincing evidence of 3-year contraceptive efficacy with Implanon, mainly by ovulation inhibition.