Congratulations on your pregnancy! I'm glad everything is going smoothly. That's great. My pregnancy went pretty well until the last trimester when I started flaring, so fingers crossed that doesn't happen to you. My only advice is to be vigilant about your UC post-delivery - mine flared really badly after I had the baby and I wish I'd been more careful about getting it under control because it ended up spiralling out of control . . .
Anyhow, this isn't to scare you - I just wanted you to avoid my mistakes because I had to stop breastfeeding at 5 months because I had to start taking really high doses of prednisone and the doctors weren't comfortable with me breastfeeding while taking so much prednisone. However, they were fine for the first five months with me taking asacol and using enemas. I did quite a bit of research on medication and breastfeeding and while some medication warnings for asacol say "don't take while breastfeeding", most experts seem to agree that it is fine as it doesn't pass through the breastmilk in doses large enough to affect the baby. I can't find the links right now, but if you google Dr. Hale and breastfeeding, you'll find a wealth of information on his website. He quite literally wrote the book on breastfeeding and medication.
31-yr old female, diagnosed with left-sided UC in 2002
1st severe flare started post-pregnancy in December 2006
2 x 400mg Asacol 3 times a day
60mg 50mg 40mg 30 mg 20mg 15mg 10mg 5 mg Prednisone a day
VSL#3, calcium + D, B-100, flax seed powder, iron, vegetarian/whole foods diet
Osteopenia (diagnosed May 2007)