I researched it a bit as I had heard that the acids (I think, it was a while ago) help with uterine toning, in the same way as raspberry leaf does. This means that it can be beneficial both just prior to conception (for implantation) and prior to labour (for contractions). That said I couldn't find any hard evidence as is the way with most natural conception (and induction) ideas. My vote would be to try it, can't hurt!

Oh & it has to be fresh pineapple, the canning or drying process removes the necessary bits to make it work. Does that make sense?