LEVONORGESTREL AS A POSTCOITAL CONTRACEPTIVE

Time-release pellets of levonorgestrel (LNG), the progestogenic hormon e contained in the contraceptive system Norplant(R), were implanted su bdermally in mice, after the animals had mated and ovulated but before uterine implantation of embryos would have occurred, to examine wheth er the hormone could reduce the number of embryos that subsequently im planted and, if so, when it had to be administered in the postcoital p eriod to achieve that effect. Hybrid female mice (C57BL x CBA) were pa ired with breeder males (CD-l) and LNG pellets were implanted on day 0 , the day on which copulation plugs were found, or on day 2 or day 3 i n the postcoital period. Mice in some groups were sacrificed on day 14 of the gestation period, and numbers of fetuses and/or resorption sit es were counted, while mice in other groups were allowed to go to term . When LNG pellets were implanted subdermally on day O of the postcoit al period, pellets designed to release 1.5 mg of hormone in 21 days fa iled to exhibit a contraceptive effect, but pellets designed to releas e 5 mg of hormone in 90 days were totally effective in preventing uter ine 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 tho se embryos that did implant. When the pellets were implanted as late a s day 3 in the postcoital period, uterine implantation of embryos occu rred and fetuses were carried to term. Results of the study indicate t hat subdermal implants of LNG inserted postcoitally can prevent uterin e implantation of embryos in mice, and thereby prevent pregnancy, desp ite fertilization of oocytes having occurred, if the hormone implants are inserted before day 3 of the postcoital period.