Levonorgestrel is widely used as emergency contraception, yet much confusion surrounds its use. Consensus statements and reviews typically attribute its efficacy to prefertilization mechanisms of action, such as suppression of ovulation and interference with cervical mucus or sperm function, yet studies do not rule out a postovulatory MOA. To yield greater clarity, the authors review recent scientific studies examining the MOAs of LNG-EC. They conclude that LNG-EC exerts minimal effects on cervical mucus and sperm function and that suppression of ovulation (...) is not the dominant MOA accounting for the contraceptive efficacy of LNG-EC. Luteal deficiencies and endometrial changes reported in the literature strongly suggest a postovulatory MOA when LNG-EC is given during the critical preovulatory period. National Catholic Bioethics Quarterly 13.4 : 677–716. (shrink)