bar some birth control methods, including IUDs and “morning-after pills” that prevent fertilized eggs from implanting in the uterus.
Contrast the Journal of the American Medical Association:
The authors of such claims [that Plan B prevents implantation] have offered no supporting evidence regarding Plan B's mechanism of action. On the contrary, what has been offered are assertions that “the exact mechanism of action of Plan B is unknown” and that “the dirty little secret is, nobody really knows.” Such sweeping statements of uncertainty not only directly contradict the claim that Plan B's anti-implantation action is “proven” but also promote general misunderstanding and misuse of doubt in science. [...]
Published evidence clearly indicates that Plan B can interfere with sperm migration by altering the cervical and uterine environment, and that preovulatory use of Plan B usually suppresses the LH surge either completely or partially, which in turn either prevents ovulation or leads to the release of ova that are resistant to fertilization. Epidemiological evidence rules strongly against interruption of fallopian tube function by Plan B. Evidence that would support direct involvement of endometrial damage or luteal dysfunction in Plan B's contraceptive mechanism is either weak or lacking altogether. Both epidemiologic and clinical studies of Plan B's efficacy in relation to the timing of ovulation are inconsistent with the hypothesis that Plan B acts to prevent implantation. [...]
[A]ll women should be informed that the ability of Plan B to interfere with implantation remains speculative, since virtually no evidence supports that mechanism and some evidence contradicts it. Women should also be informed that the best available evidence indicates that Plan B's ability to prevent pregnancy can be fully accounted for by mechanisms that do not involve interference with postfertilization events.
Thanks to Helen for informing me on this topic. Does anyone know if the claim about IUDs is similarly misleading? Links welcome!
P.S. Medical disputes aside: Is there a standing legal obligation even to let "people" implant in your uterus? Seems odd.