I had to have an emergency root canal when I was 6 weeks. This was before I even had an OB. The dentist agreed, only because of the amount of pain I was in. I was allowed general anesthetic, but not regular Novocaine. I was able to have Xrays, but they covered me with 2 of the shield they lay on top of you.
Shortly after, concerned about the amount of Tylenol I was taking for pain I scheduled an appointment with an OB. She ensured me that it was 100% okay to have that dental work done and that Tylenol, as well as Tylenol with codeine even was okay.
However, the dentist himself now wants me to wait until the 2nd trimester to get the crown put on. I think it's for liability reasons for himself, it's best to just wait unless you're in pain. Although my OB keeps telling me it's okay to go in and have the crown done, I'll still wait.