First off all, congratulations!
I had an MFM during my pregnancy for two reasons, one due to my uc and the other because I was pregnant with twins. The MFM worked closely with my OB and my gastro to determine the best course to take. I ended up on bedrest in the hospital due to early contractions which some said was due to my uc and some said due to the twins. I didn't have any problems with my uc until my 3 trimester at which time my MFM and gastro really had to work together to find some happy medium regarding meds for me to take to stop contractions that wouldn't exacerbate my uc. I was very glad to have them all on my team. It was reassuring.
I would make sure that the MFM you choose is in the same group or affiliated with the same hospital as your gastro if you are going to use an MFM.
Hope that helps and good luck!