I have already filed my return, but I saw that there is an unborn child tax credit. How do I amend and add my child?

The only state with an unborn child credit is GA.  Are you in GA?   If so you will need to amend your state return after it has been fully processed.

There is not a federal unborn child tax credit.  If you are in GA, there is an unborn child exemption, but not a credit.  This would reduce your taxable income, but not below $0.  

If this is the state you are talking about and you did not claim the exemption, you can Amend Your Return to add the exemption.  If you are talking about another state, please respond with which state?