Prenatals

I started taking a prenatal a couple of months before ttc per my doctors advice. Now I'm 4 weeks pregnant and have two vitamins left in my bottle, I want to pick up some more today or tomorrow. But should I stick to what I'm currently on? It's offered free at a local pharmacy, but I'm not sure it's the best option. I'm not sure how much of what = a good vitamin. Any advice?