Post-coital administration of levonorgestrel does not interfere with post-fertilization events in the new-world monkey Cebus apella.

BACKGROUND Experimental evidence to disprove the belief that emergency contraception with levonorgestrel (LNG) prevents pregnancy by interfering with post-fertilization events is lacking. Here we determined the effect of post-coital and pre-ovulatory administration of LNG on fertility and ovulation, respectively, in the Cebus monkey. METHODS To determine the effect on fertility, LNG 0.75 mg or vehicle were administered orally or s.c. once or twice within the first 24 h after mating occurring very close to the time of ovulation. Females that became pregnant were aborted with mifepristone and re-entered the study after a resting cycle until each of 12 females had contributed, in a randomized order, two LNG and two vehicle-treated cycles. To determine the effect on ovulation, LNG 0.75 mg or vehicle were injected twice coinciding with follicles smaller or larger than 5 mm in diameter. Six females contributed five treated cycles each. RESULTS The pregnancy rate was identical in vehicle- and LNG-treated cycles. LNG inhibited or delayed ovulation only when treatment coincided with a follicle <5 mm diameter. CONCLUSION In Cebus monkeys, LNG can inhibit or delay ovulation but, once fertilization has taken place, it cannot prevent the establishment of pregnancy. These findings do not support the hypothesis that emergency contraception with LNG prevents pregnancy by interfering with post-fertilization events.