Yes, both Implanon and Nexplanon effectively inhibit ovulation as their primary mechanism of contraception.
The contraceptive effect of both Implanon and Nexplanon (etonogestrel implants) is achieved primarily through suppression of ovulation, along with secondary mechanisms including increased cervical mucus viscosity and endometrial alterations 1.
Mechanism of Action
The etonogestrel implant works through multiple contraceptive mechanisms 1:
- Primary mechanism: Ovulation suppression - The implant prevents the mid-cycle luteinizing hormone (LH) peak necessary for ovulation
- Secondary mechanisms: Thickening of cervical mucus (limiting sperm entry) and alterations in the endometrium (inhibiting implantation)
Evidence of Ovulation Inhibition
The pharmacodynamic data strongly support ovulation suppression as the dominant mechanism:
Immediate effect: Etonogestrel levels are sufficient to inhibit ovulation within 8 hours of insertion 2. This is why backup contraception is only needed for the first 7 days after insertion 3.
Sustained suppression: Clinical studies using ultrasound and progesterone monitoring demonstrate that ovulation occurs in less than 5% of users even after 30 months of use 2, 4. In one rigorous 3-year study, the first ovulation with Implanon was not observed until after 30 months 4.
Dose-response relationship: After insertion, etonogestrel levels peak at approximately 813 pg/mL within 4 days, then reach steady-state levels around 200 pg/mL after 4-6 months—concentrations that remain sufficient to prevent ovulation for at least 3 years 2. Even in years 4-5 of use, mean etonogestrel concentrations remain in the range of 111-143 pg/mL 1, which continues to suppress ovulation effectively.
Clinical Implications
The ovulation suppression is highly effective but not absolute in all users throughout the entire duration of use. While the implant maintains contraceptive efficacy through multiple mechanisms, ovarian activity gradually increases after 6 months of use—follicle-stimulating hormone and estradiol levels approach normal ranges, though progesterone remains in the subovulatory range for over 3 years in most users 2.
Important caveat for obese patients: Women with BMI ≥30 kg/m² have relatively lower mean etonogestrel serum concentrations (ratio 0.76-0.88 compared to BMI <30 kg/m²) during years 3-5 of use 1. However, there is substantial overlap in concentration ranges between BMI categories, and no pregnancies were reported in clinical trials during years 4-5, including in women with BMI ≥40 kg/m² 1.
Rapid reversibility: Ovulation resumes promptly after implant removal—most women ovulate within 3-4 weeks, and pregnancies have been observed as early as 7-14 days post-removal 2, 4, 1. This confirms that the ovulation suppression is reversible and not permanent.
The exceptional contraceptive efficacy of these implants (failure rate 0.05% with typical use) 3, 5 reflects the combined effect of ovulation suppression plus the backup mechanisms of cervical mucus thickening and endometrial changes.