trish,
don't mean to throw a spanner in the works, but a woman at our twin club conceived her ID twins through IVF. She had two embies put back and ended up with twins. She thought there would be frat, but it turns out that they are ID. Only one emby must have stuck and it split. Apparantely there is a slightly higher risk of the embies splitting through IVF. They don't know why, but the stats say that it is more prevailant.

hmm - the human body is an amazing thing
p.s. my two are frat - one has a penis and one doesn't -lolol