prenatal vitamin question

So I was taking Pnv prenatal plus multi vitamins and folic acid daily as prescribed by my obgyn , then I switched doctors and was due for a refill and they thought that it was strange that he had prescribed folic acid and prenatals given that prenatals already have 1mg of folic acid in them. So they canceled the folic acid prescription. Why did my previous obgyn prescribe so much folic acid ? Could it be harmful?