I approached my doctor about the pill around the same time my symptoms first developed. We didn't know what was going on at that time but he said that he was concerned with absorption and we decided to go with the Evra patch. The only down side was that the patch has higher levels of estrogen and this increases the risk of clotting. At the time it didn't seem like an issue, I had no previous clotting issues, non-smoker, no family history of clotting, and was active with a healthy BMI. Unfortunately, between the increased risk of clotting with crohns and the higher than normal risk for clots with the patch, I developed a DVT. After that I was put on the mirena IUD and I LOVE it! No side effects, no periods, and no more increased bowel issues that I used to have along with my period.
I think that afidz is correct in saying its very case by case, the individual risks and benefits should be discussed with a doctor familiar with your IBD and other history, but an IUD may offer some benefits for women with IBD.