Prenatials before pregnancy?

Me and my husband arnt really trying in the sense we monitor periods, track ovulation, or make it a point to have sex during ovulation. We just have sex when we want and if it happens it happens. My doctor is suggesting I start taking prenatals instead of my normal women's vitamin to protect incase I get pregnant. Is that really necessary? I don't want to buy prenatals for however long it takes for us to get pregnant unless it really is a necessity. I deffintly plan to start taking them when we actually start trying. Can I get opinions please?