The Vatican might be correct on Plan B

A reader responds to my post of William Saletan's criticism of the Vatican's statement regarding Plan B:

Just to respond briefly to Will Saletan - there is no definitive evidence of how Plan B works.  Saletan is guilty of the same exaggeration that he criticizes in the CDF document when he says that studies have "completely wiped out" the claim about Plan B preventing implantation.  The picture is far murkier (as Saletan's quote suggests).

Originally, it was thought that Plan B could affect the endometrium in a fashion that would prevent implantation of a developng embryo.  Ling WY, Robichaud A, Zayid I, Wrixon W, MacLeod SC (1983). "Mode of action of dl-norgestrel and ethinylestradiol combination in postcoital contraception". Fertil Steril 40 (5): 631–6. PMID 6628707.  Kubba AA, White JO, Guillebaud J, Elder MG (1986). "The biochemistry of human endometrium after two regimens of postcoital contraception: a dl-norgestrel/ethinylestradiol combination or danazol". Fertil Steril 45 (4): 512–516. PMID 3956767.  Yuzpe AA, Thurlow HJ, Ramzy I, Leyshon JI (1974). "Post coital contraception—a pilot study". J Reprod Med 13 (2): 53–8. PMID 4844513.

Subsequently, some scientists have re-evaluated these studies and have suggested that Plan B might not prevent implantation.  The literature survey published in JAMA suggests as much.

Studies have also shown that, in women who ovulate despite taking Plan B before ovulation, there are changes in certain hormones such as progesterone and in the length of luteal phase.[112] These secondary changes might inhibit implantation in cases where fertilization occurs despite Plan B use.  See e.g., Durand M, del Carmen Cravioto M, Raymond EG, Duran-Sanchez O, De la Luz Cruz-Hinojosa M, Castell-Rodriguez A, Schiavon R, Larrea F (2001). "On the mechanisms of action of short-term levonorgestrel administration in emergency contraception". Contraception 64 (4): 227–34. doi:10.1016/S0010-7824(01)00250-5. PMID 11747872

The FDA's webpage indicates that Plan B might work in some cases by preventing implantation.
http://www.fda.gov/CDER/DRUG/infopage/planB/planBQandA.htm

Bottom line is that there is no demonstration that the CDF claim is false.  There are some good reasons to think that it is true.  And if you take the position of the CDF (namely, that every innocent developing human organism is entitled not to be killed), caution regarding Plan B is in order.  I wonder if the "is certainly present" that Saletan objects to is a translation problem from the Italian.