I don't have children, so I can't speak from personal experience about
carrying a baby on Imuran. But I did a lot of research because I'm trying to get information for when I do have kids. I've spoken with 2 GIs and both say that Imuran is perfectly safe for pregnancy. Their reasoning is that if you flare while pregnant, the risks to the developing fetus are much higher (fistulas, malsorption, etc) then the amount of Imuran in your blood stream.
Imuran has a category D with the FDA from a study completed in the 1970s that showed a limited sample size and HIGH (200mg+) doses of Imuran (in organ transplant patients). The study showed that several babies were born with a decreased immune system then their healthy counterparts. However, every baby in the study's immune system was able to normalize. It just took a few extra days. The reason that Imuran gets a bum rap is from that study and once you're labeled something with the FDA, it is extremely difficult to get your rating reversed.
Remicade is a category B, which means there is some risk to the fetus in animal studies. Truthfully, I think the reason that the risk is rated so low is that it's not been around long enough for enough people to have problems. It's been around for 10 years compared to Imuran's track history of 50(?)+ years. Plus, Imuran is more widely used for more conditions then Remicade.
Personally, when I sat down and thought it out. The benefits of being on Imuran/Remicade or whatever medication you are on (aside from methotrexate) far, far, far outweigh the risks. Though it's still a long, long time for babies for me.
Hope I helped!