1. After the period for rescission under section 14-20-17 has expired, a signatory of an acknowledgment of paternity or denial of paternity may commence a proceeding to challenge the acknowledgment or denial only: a. On the basis of fraud, duress, or material mistake of fact; and b. Within two years after the acknowledgment or denial is filed with the department of health and human services. 2. A party challenging an acknowledgment of paternity or denial of paternity has the burden of proof.