Welcome to the forum. Canasa is a 90 percent topical medication and 10 percent systematic. It's a mild anti-inflammatory mesalamine and is generally safe and well tolerated by most without side effects.
Here's a few links on UC and pregnancy.
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