Hi srowe! I was told by my doctor that the benefit of 6mp can outweight the risks for a Crohn's patient. That is, healthy mom = healthy baby. I think it would be worse and more dangerous to be pregnant and flaring that being healthy and pregnant on the 6mp. I've also done a lot of research and there are several small studies that show there is very little increased risk to a pregnancy. I think the drug company rates it as a "D" class drug because they haven't done any large scale studies on pregnant women (nobody does!) and they want to cover their asses.
Also, 6mp has been out on the market for decades and was originally made for treating cancer patients (for certain types of lymphoma I believe). Anyway, the doses of 6mp that a cancer patient would take are much, much higher than what a Crohn's patient takes, so there is less risk in a smaller dose.
I am definitely NOT a medical proffesional but I just wanted to share with you what I've learned from lots of research and my doctors. I have read of many people having very normal pregnancies while on 6mp. You may want to check out about going to a Fetal-Maternal Specialist (that deal with high-risk pregnancies) to get their advice as well.
Personally, I'm planning on staying on the 6mp when I get pregnant. It's a very tough, personal decision to make but in the end, it is your decision.
Best of luck to you!