This is from my limited knowledgebase. There are 4 major OB/GYN fellowships: gyn oncology, maternal fetal medicine (perinatology), urogynecology/pelvic surgery and reproductive endocrinology and infertility. If you go to NRMP web site you can find out how many spaces are available nationwide. All fellowships are very competitive, except for MFM (right now). Gyn oncologist, I think, have the worst lifestyle with lots of hours and emergencies. Urogyn, MFM and REI have much better lifestyle. There are also fellowships in adolescent and pediatric gyn, laporascopy, sexuality, and some other odd ones. Hope this helps.
Dani