Welcome to the forum!
We've had a number of moms take Pentasa while pregnant and have happy and healthy babies without issues. Pentasa is a mild, 90% topical, anti-inflammatory medication and well tolerated by most. It's important for mom to be healthy during the pregnancy to have the best outcome for her baby, so I would strongly recommend taking your medications throughout your pregnancy. You're more likely to have complications by foregoing the meds and causing a UC flareup.
Here's a couple links from our Resources section that you might find helpful:
From conception to delivery: Managing the pregnant inflammatory bowel disease patientwww.ncbi.nlm.nih.gov/pmc/articles/PMC3974516/
The Second European Evidenced-Based Consensus on Reproduction and Pregnancy in Inflammatory Bowel Diseaseecco-jcc.oxfordjournals.org/content/eccojc/9/2/107.full.pdf
Moderator Ulcerative Colitis
John, 39, UC Proctosigmoiditis
Rx: Remicade @5mgs/kg/6wks; daily 75mgs 6MP, 4.8g generic-Lialda, and rowasaU.C. = Unimaginable Crapnado