I would talk to your doctor.
I did a little research and rhubarb is sometimes used as an herbal preparation to treat pregnancy induced hypertension. It is also thought to be a uterine stimulant (is one of those foods that will allegedly help bring on labor in women close to term).
So if you do eat it, do so in moderation :)
I'm not too sure. I think I read somewhere once that it's not, but I can't think of it at all. I would call your doctor and see what they say. I'm sure they'll have a more clear answer.