Uceris oral is a topical released into the colon...but I was thinking if steroid is helping, then that is another option. I was also wondering if it's possible you're sensitive to mesalamine...therefore replacing it with a topical steroid and being off the mesalamine might give that answer.
Have you had B vitamins checked while pregnant?
Fibre supplements...im only suggesting to add a small amount to help form stool, especially while eating a low residue diet. There are different forms...https://www.healthline.com/health/digestive-health/whats-the-best-fiber-supplement#psyllium