Most Helpful Girl
Get a DNA test and see if the child is yours. Hopefully it's not.
Most Helpful Guy
That varies by state, you need to consult a family law attorney near you.
Under Georgia law, if you're married to a woman, any child born to the wife is automatically the child of the father unless a court declares otherwise. I am not sure if that applies to whether the child was conceived during the marriage.
Though you didn't say if you were married or not. If you weren't married, as long as your state does not have common law marriage, tell your ex to go pound stand. If your name was not on the birth certificate and there was no marriage, unless you filed to make the child legitimate, you should be fine.