I don't think many if any dentists use mercury fillings any more. Mercury was phased out a long time ago.

BTW when pregnant it is super important you look after your teeth and gums and have regular check ups. The danger to the baby of the flow on effects of gum disease far outweighs the unsubstantiated risks of things like fluoride. Before I fell pregnant I'd just finished intensive treatment for a specific gum issue and my dentist said that the toxins, bacteria etc produced through gum disease can do harm. So dental visits are more important than ever.