Ulipristal acetate (UPA), a progesterone receptor (PR) modulator, is used as an emergency contraceptive in women. Here, using a mouse model, we investigated the mechanism of action of UPA as an ovulation blocker. In mice, ovulation is induced ~12 hours following the treatment with exogenous gonadotropins, including human chorionic gonadotropin (hCG), which mimics the action of luteinizing hormone (LH). When administered within 6 hours of hCG treatment, UPA is a potent blocker of ovulation. However, UPA's effectiveness declined significantly when it was given at 8 hours post hCG. Our study revealed that, when administered within 6 hours of hCG, UPA blocks ovulation by inhibiting PR-dependent pathways intrinsic to the ovary. At 8 hours post hCG, when the PR signaling has already occurred, UPA is unable to block ovulation efficiently. Collectively, these results indicated that UPA, when administered within a critical time window following the LH surge, blocks PR-dependent pathways in the ovary to function as an effective antiovulatory contraceptive.