In no state will you find a law requiring back child support if the identity of the biological father has not yet been legally established.  While, if the child was still under 18 years of age, the father, once notified legally, would have to pay future child support payments until that child was a legal adult. In some states, I think the age is 21 if they are still dependents.  But the father would never have to pay back child support, as no child support payments had yet been agreed upon or required by a court.  Therefore, how could payments have been missed?