Thats a hard one. You dont want to create to much toxins with antifungals while pregnant. You might just have to stick with a very strict diet during your pregnancy. Thats not easy to do when cravings come about.
Usually for most people it takes more than just mercury fillings to develop strong candida. Think of how much of society has mercury fillings and they dont have severe candida. Antibiotics will have more of an impact on candida than mercury fillings. Although Mercury fillings dont help with candida. Mercury is toxic to all cells including candida. Jeff mccombs has a good clip on this.
You can ask Dr. Jeff and see what he might have to say.