The only one I know might be bad in too high a quantity is vitamin A. Here's text from Wikipedia's page on vitamin A:
"Toxic effects of vitamin A have been shown to significantly affect developing fetuses. Therapeutic doses used for acne treatment have been shown to disrupt cephalic neural cell activity. The fetus is particularly sensitive to vitamin A toxicity during the period of organogenesis."
Vitamin A - Wikipedia, the free encyclopedia
I think my prenatal vitamin (which is Safeway's generic brand, has a lot of folic acid, etc.) only has 50% of the RDA of vit. A. and it's in the beta carotene form (as opposed to retinoid form) which is for some reason supposed to be less harmful.