For those of you who do not think that birth control pills or emergency contraception (basically any drug with progestin in it) can cause abortions by preventing implantation at times, please please read this article:
or for a shorter version of the article, go here:
It cites many peer-reveiwed journals as sources (scroll down to the bottom to see), as well as the Physicians' Desk Reference, which is the best source of info to use for understanding the mechanism of any drug since doctors use it! :-) Public libraries should have copies of it, too, if you are interested in going to one and looking the drugs up in it. Happy reading.