The birthday rule is a common claims practice health insurance companies use when children are listed as dependents on two parents' group health plans. The rule helps determine which health plan is the primary policy and which is secondary so that total coverage does not exceed 100 percent of charges.

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According to the birthday rule, the primary plan belongs to the parent whose birthday falls closest to the beginning of the year. The ages of the parents are irrelevant when determining the birthday rule. The primary plan pays out a benefit first and the secondary plan pays the remainder of the medical cost if it is a covered benefit on the secondary plan.

It’s important to remember that the "birthday rule" is not a law -- it is simply a practice that many insurers follow. Other insurance companies do not use the birthday rule.

Here is how the birthday rule typically is modified in some other situations:

Same birthday. If you and your spouse share the same birthday, the policy that has provided coverage the longest becomes the primary plan.

COBRA vs regular health insurance. If your spouse has COBRA insurance through a prior employer and you have a health insurance plan through your current employer, your plan is the primary, regardless of the birthday rule.

Legally separated or divorced and not remarried. The custodial parent's plan is the primary and the noncustodial parent's plan is secondary. However, if one parent has a group plan and the other has an individual plan, the group plan becomes the primary plan.

Divorced and remarried. The custodial parent's plan is the primary, the stepparent's plan is secondary. If costs still remain, the noncustodial parent's plan pays last.

It's important that you read your health insurance policies carefully and work with your insurance companies to understand how benefits are coordinated. Not all plans follow the birthday rule, so it's a good idea to determine which plan is the primary and which is the secondary before you start incurring medical costs.