45 CFR 303.106 - Procedures to prohibit retroactive modification of child support arrearages.
(a) The State shall have in effect and use procedures which require that any payment or installment of support under any child support order is, on and after the date it is due:
(1) A judgment by operation of law, with the full force, effect, and attributes of a judgment of the State, including the ability to be enforced;
(2) Entitled as a judgment to full faith and credit in such State and in any other State; and
(3) Not subject to retroactive modification by such State or by any other State except as provided in paragraph (b) of this section.
(b) The procedures referred to in paragraph (a)(3) of this section may permit modification with respect to any period during which there is pending a petition for modification, but only from the date that notice of such petition has been given, either directly or through the appropriate agent, to the obligee or (where the obligee is the petitioner) to the obligor.