Pentasa is a 90 percent topical medication, it's a mild anti-inflammatory medication that's well tolerated by most without side effects. There have been many moms here who've taken the same medication you are taking and had happy and healthy babies. It's important that mom take her medication to control her UC to have the best outcome for her child. If you were to stop taking your medication then yoyr UC could flare up again, you could become malnourished and deliver your baby earlier than expected and have potential problems as a result.
Here are some good reads from our resources section:
From conception to delivery: Managing the pregnant inflammatory bowel disease patient
www.ncbi.nlm.nih.gov/pmc/articles/PMC3974516/The Second European Evidenced-Based Consensus on Reproduction and Pregnancy in Inflammatory Bowel Disease
ecco-jcc.oxfordjournals.org/content/eccojc/9/2/107.full.pdf