This is a topic I've been struggling with for awhile. If you search, you'll find numerous threads. Some say it's safe, some don't...I say trust your doctor. Mine said not to take Imuran during pregnancy and wants me on prednisone instead. To my knowledge, nobody has every actually posted saying "I took Imuran and it definitely affected my baby."
Just my two cents...if I can control the potential flares without potentially harmful drugs during pregnancy, you can bet I'll do it. Also remember that timing is crucial. If you're in remission when you conceive, supposedly you have a much better shot of staying that way throughout the pregnancy.
I wish you the best with whatever you choose! I know it's scary!
P.S. I just noticed you're another Molly. Whoohoo!