Wouldn't another option be to do an Ob/Gyn residency then subspecialize in maternal-fetal medicine, since that's all high-risk obstetrics? If I go Ob/Gyn, I'm making eyes at REI or MFM if I decide to subspecialize.
To the OP, to answer the "vice versa" part of your question, a lot of Ob/Gyns filter out the Ob side of their practice. Some do this for the hours, or for a drastic reduction in malpractice rates (which I think is often the case in my area), or for other reasons, but they do it.