Durand's group biopsied the uterine lining and did not find any thinning or other changes in the lining, except that some women had an increase of glycodelin A, which interferes with fertilization and increases implantation.
Take a look at that "LTI" link. Larimore, Kohlenberg and Sanford, in their Archives of Family Medicine article reproduced at polycarp.org,discussed the strength of evidence of post fertilization effects of the various hormonal contraceptives that are taken every day.
In fact, Plan B has not shown any positive implications as an abortifacient:
Post-coital administration of levonorgestrel does not interfere with post-fertilization events in the new-world monkey Cebus apella. Hum Reprod. 2004 Jun;19(6):1352-6.