Hopefully Siri will see this and answer your questions about OB/GYN NPs. I am not sure if that certification is available anymore.
The education for a CNM in my program was 5 semesters full time - I was in a master's degree in nursing program. Two semesters were classroom (or online) courses, mostly courses taken with all the master's students from all the various NP/CNS programs, things like nursing theory, advanced assessment, and advanced pharmacology. Two semesters were a mix of classroom and clinical experiences in prenatal care, postpartum care, primary care of women, intrapartum care (labor and delivery) and newborn care. The last semester was "integration", which was a full-time internship with a CNM practice.
To be accepted to a CNM program, you need to have a BSN (or an ADN if you want to apply to a RN-MSN program). Most programs require some bedside nursing experience, usually L&D experience. There are also some master's direct entry programs for CNM's for people with a bachelor's degree in something other than nursing, as well as some DNP programs.
More information about CNM education can be found on the American College of Nurse-Midwives website -
http://www.acnm.org/careers.cfm?id=80
Becki