Reproductive potential and implant loss in female hamadryas baboons (Papio hamadryas) previously contracepted with melengestrol acetate contraceptive implants at AZA institutions.

Melengestrol acetate (MGA) implants are a progestin-based reversible contraceptive used to manage fertility in animals. MGA implants are recommended for replacement every 2 years; however, reproduction may be suppressed longer if implants are not removed. In this study, we investigated whether the probability of reproducing (pR) differed among nonimplanted females, females with MGA implants removed, and females whose implants were not removed. In addition, since implant loss in hamadryas baboons is a concern, we explored whether female age, institution, implant placement year, implant location, or implant placement type (intramuscular vs. subcutaneous) differed for females whose implants were lost compared to those that were not. The pR differed significantly across all three treatment conditions with the nonimplanted group having the highest pR. The pR plateaued at 63% after 40 months for the implant-removed group compared to 96% after 84 months in the nonimplanted group. There was no reproduction after contraception if implants were not removed (7.83-45.53 months). In the nonimplanted group, pR was significantly higher for older and parous females. In terms of implant loss, we found that implant placement type was significantly associated with implant loss, such that there were fewer losses when implants were placed intramuscularly (IM) as compared to subcutaneously. Our results suggest that placing MGA implants IM is likely to reduce loss. When loss is prevented, MGA implants are an effective form of contraception and are reliably reversibly in most individuals when removed. However, if not removed, they can prevent reproduction longer than 2 years.