Pregnancy while on Imuran?

Im 30 years old and have no children. But I have severe ulcerative colitis and I've been taking Imuran for 7 months I take 50 mg a day and just found out I have to take it for life because this is the only thing keeping me in remission from a flare up. I talked to my GI Dr. about pregnancy while taking Imuran and she said that it is safe to take and it is a class B drug. But if I get off Imuran if I do become pregnant she said I have I high chance of a miscarrage and birth defects because of my condition. I have alos done some research about pregnancy ad Imuran and other women have had healthy babies with no effects. I recently saw my Gyne today and he asked me about taking Birth Control because of this drug but I just want to know what you suggest