Legal state statute to determine coordination of benefits for primary and secondary carriers of dependent children covered under both parents’ insurance plans. The health plan of the parent whose birthday (month and day, not year) falls earlier in the calendar year pays first, and the plan of the other person covering the dependent is the secondary payer. If both policyholders are born on the same day, the policy that has been in force the longest is the primary policy. Birth year has no relevance in this situation. Also called birthday rule .