To me, that's kind of silly. With my last pregnancy I developed an aversion to my vitamins (all vitamins actually, it was the smell) because I had HG. Folic acid and iron are the most important components of prenatals and they are ridiculously easy to supplement through diet. If you're eating healthy, plenty of dark leafy greens and lean protein, you most likely have healthy levels of folic acid and iron. When I developed my aversion my OB had me choke down a children's chewable with iron (the fruity smell covered up enough of the vitamin smell for me to be able to take it) and eat a bowl of iron adn folic acid fortified breakfast cereal. My daughter is now a strong, healthy, intelligent 4 year old
When you say she prescribes, do you mean that she hands you a prescription to be filled at the pharmacy? That sounds like she's getting a kickback because my OB insisted there was no reason to buy prescription prenatals because the over the counter varieties (even generics) are just as good. Your doctor is encouraging you to waste money.