How does this HMO thing work?

So I have a normal aetna plan right now. As of Jan 1 it will be switched to Aetna HMO. Now I am right in the middle of fertility treatments with my OBGYN. I have read that you need to go to a specific doctor in order for my insurance to work. Well I picked a regular family doctor for my plan because it required one. But what about my OBGYN doctor? She said she accepts all Aetna plans so it is still good but will it be okay if I continue going to her after the new year since I haven't been referred? Or will have to make a trip to my new general practitioner first and then get her to refer me to my old OBGYN? I am confused on this whole thing.