I took 150 mg of Imuran throughout my pregnancy with my almost 4 year old. There were no problems whatsoever. I also started out on 150 mg Imuran when I got pregnant with my 10 month old - I was also on Humira and was able to go off the Imuran about 8 weeks in (pregnancy does REALLY good things with my Crohn's) and just stayed on Humira the entire time. There were no problems with her either.
My OB wanted me off Imuran before I even tried to get pregnant. I tried that and had a massive flare and ended up in the hospital. After discussing it with my GI, he deferred to my GI's judgment that it was fine and here I sit with a preschooler and a baby.