Contraceptive or Abortive?

Clearly as Catholics, we're not supposed to be doing either one, nor do I have any desire to, but in regards to questions of conscience and regulation the facts of the matter are clearly important. It's been discussed often in pro-life circles over the years that oral contraception is designed such that it sometimes allows fertilization but prevents implanation -- thus in effect causing a spontaneous abortion several hours after conception. This is generally applied even more so to "morning after" pills, which is a reason why some pharmicists have conscience objections to dispensing such medications. Given that that Protestant half of the pro-life movement is often fairly comfortable with birth control in concept, this "the pill causes abortion a certain percentage of the time" argument has often been used to help unify the pro-life movement against birth control.

Regardless of whether it's true, I think that clearly the contraceptive mentality and the approach to sex it entails is certainly a major cause of support for abortion.

All that said, I'd very curious as to the reaction of Catholics more educated in the precise medical details of human reproduction than I am to this Slate post, which argues that the evidence for abortificant properties to The Pill and Plan B is slim to none.

I've no particular interest in endorsing Saletan's opinions generally, he's the one who made the to my mind rather nutty argument a while back that Planned Parenthood was overall an anti-abortion organization because contraception prevents unwanted pregnancies. But if it's correct as a matter of medical science that it's very, very rare for oral contraceptive and even "morning after" pills to cause spontaneous abortions, we'll do nothing but make ourselves look unconcerned with the truth (and the scientific truth of embryology is very much on our side when it comes to what the pro-life movement says about abortion as opposed to what the "just a clump of cells" people say) if we keep pushing it.

Feedback from those with medical or scientific knowledge of the issue would be appreciated.