L Lysine is an amino acid supplement used to help inhibit the herpes virus, but it is classed as catergory C, and not recommended during pregnancy, but this is because they don't know the effects of it during pregnancy and breastfeeding.

Eating yogurt is supposed to help because the active culture helps fight the virus, and treat and prevent cold sores.

I've always used eucalyptus oil on a cold sore when I first notice it coming up, and it doesn't spread or turn into a big blister. But I don't know if you can do that while pregnant? I'll have to have a look... I might have to find a new cold sore treatment.