It should be safe but as genamarie said, ask your doctor. I have read online and heard that too much vitamin C can cause miscarriage and early labor, but I am not sure if that is just a rumor or not. But then again that was like 1000mg a day.
This is really a question for your doctor, as they should be the only one making medication recommendations for you, but as far as vitamin C and pregnancy, it should be safe, I'd check with your doctor especially if you are on any other meds that could be effected by the excess vitamin C before you start any.