Shan, my understanding is that there are no studies to prove that it doesn't cross into BM, so as a precaution, you're asked to wean. As far as the effectiveness of the cycle goes though, it's hard to say unless you can compare with cycles where you're not BFing, but again no large scale studies done in that area that I know of, and with the majority of women asked to wean before IVF, I bet there's very little anecdotal evidence as well.