Researches over many years have examined whether levonorgestrel emergency contraception has a postfertilization effect. In a recent article in the Catholic Health Association’s journal Health Progress, Sandra Reznik, MD, asserts that “levonorgestrel acts to prevent pregnancy before, and only before, fertilization occurs.” A companion article by Ron Hamel, PhD, argues for the moral certainty that Plan B is not an abortifacient. Reznik fails to address the principal model supporting a potential postfertil­ization mechanism of action, specifically, that preovulatory administration of levonorgestrel (...) disrupts the delicate ratio of estrogen and progesterone essential to healthy endometrial development and induces the equivalent of luteal phase insufficiency, thereby jeopardizing implantation. Hamel’s argument for moral certitude is similarly inadequate. This article critically reviews both articles and the sources on which they rely. National Catholic Bioethics Quarterly 10.4 : 741–772. (shrink)