Just take the over-the-counter ones. My doc says the prescribed ones aren't any different than the ones you get from the store, so if your over-the-counter ones have more iron just take those. It is possible to get too many vitamins. If you really feel like you need even more iron, you should ask your OB what you should do, they can prescribe you an iron supplement.
It wont hurt the baby, but you may get (I believe) kidney stones. I know when your body takes in too many vitamins they'll come together and get removed from the body. If you're not getting enough iron, you should take an iron supplement, not another prenatal pill.
I agree with sehale 100%! I van testify first hand of taking to many vitamins and developed kidney stones this was before I was pregnant....and the problem never goes away! I have yo take a special prenatal vitamin that doesn't have calcium in it! I personally would ask your doctor if you need an addition iron pill...if you do...just take the additional iron pill with the regular prenatal vitamin....that way you wouldnt over load your body! Best of luck dear!
Sorry about the spelling darn autocorrector on my phone is being funny!