Mechanism of Action of ECPs

A recent study found that most often, ECPs reduce the risk of pregnancy by inhibiting ovulation (Marions, et al., 2002). A more recent study suggests that ECPs only work by preventing ovulation or fertilization, and have no effect on implantation (Croxatto, et al, 2003). Below is a link to the most recent research:
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The following was written by internationally renown EC expert, James Trussell, PhD. Princeton University Office of Population, September 1998.

Several clinical studies have shown that emergency contraceptive pills (ECPs) can inhibit or delay ovulation. Some studies have shown histologic or biochemical alterations in the endometrium after treatment with the regimen, leading to the suggestion that ECPs may act by impairing endometrial receptivity to implantation of a fertilized egg. However, other studies have found no such effects on the endometrium, and it is not clear whether the endometrial changes that have been observed would inhibit implantation. Additional possible mechanisms include interference with corpus luteum function; thickening of the cervical mucous resulting in trapping of sperm; alterations in the tubal transport of sperm, egg, or embryo; and direct inhibition of fertilization. No clinical data exist regarding the last three of these possibilities. Nevertheless, statistical evidence on the effectiveness of ECPs suggests that there must be a mechanism of action other than delaying or preventing ovulation. ECPs do not interrupt an established pregnancy, defined by medical authorities such as the United States Food and Drug Administration/National Institutes of Health and the American College of Obstetricians and Gynecologists as beginning with implantation. Therefore, ECPs are not abortifacient.