Is Plan B different than other forms of birth control?

"Reader 1," who has defended the Vatican's seemingly confident assertion that Plan B prevents implantation, is having second thoughts.  The reader recommended that I look at this recent report by Princeton researchers suggesting that Plan B may inhibit implantation, but the report also includes this statement:

To make an informed choice, women must know that ECPs—like all regular hormonal contraceptives such as the birth control pill, the implant Implanon, the vaginal ring NuvaRing, the Evra patch, and the injectable Depo-Provera, and even breastfeeding—may prevent pregnancy by delaying or inhibiting ovulation, inhibiting fertilization, or inhibiting subsequent implantation of a fertilized egg. At the same time, however, all women should be informed that the best available evidence is consistent with the hypothesis that Plan B’s ability to prevent pregnancy can be fully accounted for by mechanisms that do not involve interference with postfertilization events.

I asked whether there is greater uncertainty regarding the inhibition-of-implantation effect of Plan B than there is regarding the birth control pill or even breastfeeding.  Reader 1 responded:

I don't know how to quantify uncertainty in this context - for one thing, it is virtually impossible to study empirically anything about the failure of implantation, for obvious reasons. It may be the case that the effect on the endometrium is the same with regular birth control. I'm not sure about breast feeding - I know that breast feeding delays the onset of menses. One interesting thing about breastfeeding is that most women discontinue it when they are trying to conceive (because of its effect on ovulation/menstruation). And those nursing women who are not trying to conceive abstain during fertile periods (if they are abiding by Catholic teaching on artificial contraception) or use other means of birth control. So I don't think, as a practical matter, it is often the case that breastfeeding would alter the endometrium in a way that prevents the implantation of a conceived embryo.

But I am actually starting to think that Plan B's postovulatory effect may be less significant than I previously thought. This might suggest that taking Plan B after ovulation is not helpful to avoid pregnancy. Women who know that they have ovulated already thus have no reason to take Plan B. Which is ironic because this is the outcome for which the CDF instruction was arguing!

Bottom line for me is that I am more comfortable with the notion that Plan B does not prevent implantation (though I think that we should continue to study its function as far as is possible). Which is a relief for me, being one who cares very much about the lives of human beings at the embryonic stage of development.

At the very least, I think that the CDF document's language (i.e., "certainly present") in this context was regrettable. At best it was unclear, at worst it is an exaggeration.