Congratulations on your pregnancy! I have taken Mezavant (another 5-ASA) at a maximum dose throughout my pregnancy. I will be 37 weeks tomorrow so the baby could arrive any day!
I was told that nearly all Crohn's medications (and UC meds too) are safe in pregnancy with the exception of Methotrexate.
I have had some minor issues related to Crohn's in the pregnancy like ongoing anemia that I needed regular iron infusions for (iron needs are much higher in pregnancy) and low weight gain. But the baby looks completely healthy and is estimated to be an average size. I had a flare at 32 weeks and was put back on Prednisone to control it fast. I am feeling much better now. It is much more risky to the baby to have uncontrolled inflammation than it is to take steroids or 5-ASAs or biologics or Aza/5-MP, etc. They are also all believed to be safe during breastfeeding.
Make sure you have a good GI doctor and a good OB (or midwife plus consultation with an OB now and then). A lot of people with IBD feel better during pregnancy that they normally do because pregnancy hormones naturally suppress the immune system.
Good luck!