CONGRATULATIONS on your pregnancy!!!! That's wonderful news!
Here's what I can tell you: Imuran is basically the same drug as 6mp. My GI in Miami who was part of a very large research team told me that the most recent studies show that women who take 6mp during pregnancy have NO GREATER INCIDENCE of birth defects/complications/abnormalities than women who DON'T take 6mp during pregnancy. He told me that while it was previously thought to be dangerous, it is proving to be "safe". He encouraged me that if I were to become pregnant, to keep taking the 6mp. I never found myself in that situation, BUT I was very relieved to know that there is medical data to back it up.
As far as breastfeeding goes - this is the only information I could find. Dr. Hale is the leading expert on medications and mother's milk. The link is actually about 6mp, but if you read all the way through he comments on Imuran and how it affects breast milk.
This one also discusses Imuran and breastfeeding:
If you can get a hold of a copy of his book your doc can read it and explain it more.
Hope this helps a little.