Currently OB/GYN has 3 "surgical" sub-specialties, 1 of which has board certification (GYN/ONC): UroGyn (Pelvic reconstruction), MIS (laparoscopy), and GYN/ONC. The latter is of course related to malignant processes and I'm sure you can imagine the scope of their practice.
The former 2 (MIS, UroGyn) serve as consultants. The bulk of their practice involves performing the consultation (after referral from a generalist whether it be an OB/GYN, FP, IM), surgical/medical intervention as needed, and necessary long term follow up. It is however good practice by consultants to follow the patients with the generalist or send the patient back after the clinical problem has been resolved. Therefore the "routine" GYN care is generally still performed by the primary MD and not the subspecialist. Now bear in mind that at times, the patient chooses to stay with the subspecialist for routine care also...
MFM and REI, generally see the patients and send them back to the generalist after the problem has been addressed (i.e. pregnancy, infertility). Hope this helps answer your question.