Generally cold sores attack when you are run down, stressed or not taking adequate diet. So yes very common during pregnancy. I am not sure if you can take or use anything best ask pharmacist HOWEVER I rub vegemite into cold sores when I get them. (pregnant or not) I believe the vitamin B is really good for them (and mouth ulcers) hurts like a bi@#ch (for 30 sec) but they are usually gone within a day or two.