I can't remember the name of my prenatal vitamin...think I bought it at Costco...but mine says to take it preconception, while pregnant, and while BFing...don't think it can be bad. The one thing that I know can be bad is too much iron, but that's anytime. Some other vitamins and minerals can be bad is taken too much, but usually a mulitvitamin (prenatal or otherwise) won't give you too much of them, even with your eating them as well.
Hope that helps (it's a little scatterbrained, sorta like me today!)