1. Wait until your second year to decide on the specific fellowship, as you'll have adequate exposure to all subspecialties by then.
2. If MFM (or another field) interests you, then by all means go for it.
3. The single biggest factor, in my opinion, is research. Pick up something retrospective, get it done and published, throw a couple of case reports in there, and you should be good.
4. The next biggest factor will be your letters of rec. If you don't have well know folks at your program, be sure to set up an elective in MFM at an outside institution your 3rd year. Secure a letter and/or interview!
Lastly, don't worry about the IMG factor much... If you want it badly enough, and work hard enough, you'll land a fellowship.
Best of luck!