The reason you take a prenatal vitamin is (mostly) for the folic acid. You also want to make sure you are as healthy as possible when ttc.
I am taking a Costco brand prenatal vitamin, as well as extra folic acid (they've just recently recommended a higher dose then in the prenatal vitamins, and it may help with depression), a vitamin B-50 complex (supposed to help with depression), and 1 (so far, may up to 2 or 3) Omega 3-6-9 (supposed to help with depression and also with baby's brain development, and I don't eat enough fish, and you're supposed to limit the amount of the fishies with the fatty acids when pg due to possible mercury content).
I am taking all of these with my doctor's knowledge, but I started taking them before even seeing my doctor.
I'd recommend a prenatal vitamin just for good health, unless you get all you need from what you eat (but let's be realistic, who really does? Not me!

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