Is it true that Janet Fraiser would have come in as a Captain because she is an MD?

Also, could she be in the military early on, leave, and then come back? I'm trying to account for a gap in service.

I'm signed up for a Janet fic and want to verify my facts.

This site is awesome BTW. Though I work near an AFB, I can't exactly ask these questions. My guess is that they all know who Janet Fraiser is, though. (g)