I am 23 weeks along, and been on Asacol and Imuran the entire time. I flared quite a bit early on (I was in remission when I was TTC), but used suppositories and enemas to help get it under control. Since the start of the second trimester, I've been doing better. My OB and peri were both OK with the meds, it was my GI who has been nervous about it the whole time. He doesn't like 6MP drugs during pregnancy. He did have me start FloraQ probiotics, and I swear that has really helped.
The key is to get healthy. See a peri to have extra ultrasounds (I had one at 12 weeks, one at 18 weeks, all was good, so no more until 32 weeks). The flares are far more dangerous than the drugs ...
Oh, but definitely talk to your pediatrician about the breastfeeding. 6MP drugs pass through breast milk, so it's a no-no.
Good luck, and congrats on your pregnancy.