It's far more dangerous to get in a serious flare while pregnant than the risks from the meds.
Before we decided to start trying for a baby, i went to a high risk OB, and saw a man who had been doing it for years and had lots of experince with meds and the risks.
I was on immuran at the time, and he said, while it is classed as a D, that it wasn't as bad as it was made out to be, that even though there were lots of drugs classified as D, only a few were really that bad, he didnt tell me what they were.
He thought it best for me to stay on the meds, it can take up to 6 months for immuran/6mp to get out of your system, and of course in those six months you can either flare, or if you get pregnant its not going to matter, the possibilites of birth defects are in the spine and brain, which are formed in the first few weeks.
The most important thing he told me was to go ahead and start taking a prenatal, or a flintstones vitamin, AND to take folic acid! the full recommended amount. Folic acid, and other Bvitamins are very important to brain development. The thing that is the most common from 6mp (according to the dr) is a cleft pallete, which compared to what could happen in a flare, is no big deal.
What most people worry about is spinabiphida (sp?) and i cant remember the exact numbers but this will give you an idea.
Every woman, regardless, has a .01% chance of having a child with that. A woman taking one of those drugs, has like a 5% chance or something like that. however, taking folic acid, before you get pregnant and during the pregnancy, takes it back down to like 1%. Now those might not be the exact numbers, but you get the idea.
I also have to take pheonobarbital which is considered a class D drug now, with the same problems, but for years was used to prevent early labor.
Anyway, long story to get to a short point, i was on immuran my entire pregnancy, and i have a perfect little boy, he is super smart, no problems at birth, none that had anything to do with that anyway, and no problems now!
Its just very very important that you take that folic acid, and make sure that you see a high risk OB who specializes in meds, and that your high risk OB, your GI, and the one specializing in meds, if its a different one, stay in contact!
good luck!