I became pregnant with my second child after being diagnosed with Crohn's. My disease was fairly inactive until I became pregnant. It started at about 5 mos and continued throughout. After I had him, it subsided. I did have to take prednisone for a few weeks because I was becoming a little dehydrated. I believed worse complications would arise if me and the babe weren't getting the nutrients we needed (because everything was moving through me too fast). Most nutrients are absorbed through your small intestine and if they are moving through too fast, not much is absorbed for you or the baby. I took the steriods for a few weeks and it was just enough to control my immune system, then didn't really have to take them anymore. You might try a week just to see if it helps.
I had my first ulerative colitis flare up when I was first pregnant and didn't know it. I had just gotten married and we were planning on waiting a couple of years for kids so didn't realize I was pregnant and had all the x-rays, etc. without knowing. This was 24 years ago. I took Asacol but no prednisone since it wasn't that bad of a flare up. I did lose that baby at the 16 week point due to the tests I'd had. I then wanted a baby so waited 2 months and tried again. My colitis was better when I was pregnant but I did continue the Asacol. I can't answer you about the prednisone. You will need to talk to a doctor. I don't know if pregnancy really has any effect or not. I think initial flare ups happen when we are stressed and for me that was the stress of a new marriage.
I did have colitis for 10 years and then had the j-pouch surgery so am now cured. Good luck with everything. Don't be afraid to ask for 2nd opinions.