Levonorgestrel as a postcoital contraceptive

Time-release pellets of levonorgestrel (LNG), the progestogenic hormone contained in the contraceptive system Norplant, were implanted subdermally in mice, after the animals had mated and ovulated but before uterine implantation of embryos would have occurred, to examine whether the hormone could reduce the number of embryos that subsequently implanted and, if so, when it had to be administered in the postcoital period to achieve that effect. Hybrid female mice (C57BL x CBA) were paired with breeder males (CD-1) and LNG pellets were implanted on day 0, the day on which copulation plugs were found, or on day 2 or day 3 in the postcoital period. Mice in some groups were sacrificed on day 14 of the gestation period, and numbers of fetuses and/or resorption sites were counted, while mice in other groups were allowed to go to term. When LNG pellets were implanted subdermally on day 0 of the postcoital period, pellets designed to release 1.5 mg of hormone in 21 days failed to exhibit a contraceptive effect, but pellets designed to release 5 mg of hormone in 90 days were totally effective in preventing uterine implantation of embryos. Although the 5 mg pellets did not prevent embryos from implanting in all cases when administered on day 2, they prevented pregnancies from going to term by causing resorption of those embryos that did implant. When the pellets were implanted as late as day 3 in the postcoital period, uterine implantation of embryos occurred and fetuses were carried to term. Results of the study indicate that subdermal implants of LNG inserted postcoitally can prevent uterine implantation of embryos in mice, and thereby prevent pregnancy, despite fertilization of oocytes having occurred, if the hormone implants are inserted before day 3 of the postcoital period.