HB 1671 revises laws related to establishing parentage and the duty to pay retroactive child support. It creates a presumption that retroactive child support is reasonable and in the child's best interest from the earliest date of conception, as determined by medical or other evidence. Courts are also empowered to order additional retroactive support for prenatal and postnatal healthcare costs. The bill allows proceedings to determine parentage to begin before the child's birth, with actions like genetic testing permissible under specific conditions. It modifies procedures for acknowledging and contesting paternity, ensuring retroactive child support is addressed in parentage orders rendered after its effective date.