I agree with lisa, castor oil is a class x medicine to take while pregnant, as in being one of the worst medicines to take, too many risks with it, I wouldn't do it.
It is a strong laxative which can cause your babys bowels to open which the baby can then swallow and cause poisoning, so the advice is to not do it. I have known people who have done it and baby was fine but I personally think it is not worth the risk. It doesn't always bring on labor either.