One of my good friend's best friend conceived through IVF and she went on to have periods through her entire pregnancy. When it first happened she was sure she had miscarried but low and behold she hadn't. Can't remember the exacts of what she told me, but I remember something about her having a heart shaped uterus or something like that so her body went on to have normal cycles during the pregnancy without it interferring with the baby.
Still would ask the dr about it, probably different for everyone. =]