What Is Coordination of Benefits?
Coordination of benefits (COB) applies when you're covered by more than one health insurance plan — for example, your own employer plan plus your spouse's employer plan. COB rules determine which plan pays first (primary) and which picks up remaining costs (secondary).
Common Dual Coverage Scenarios
- Spouse's plan + your own plan: Your plan is primary for your care; spouse's plan is secondary
- Parent's plan + your own plan: If you're under 26 on a parent's plan and also have your own, your plan is primary
- Two working spouses: Each person's own employer plan is their primary; the spouse's plan is secondary
- Medicare + employer plan: Depends on employer size — for 20+ employees, employer plan is primary
How COB Reduces Your Costs
When you have two plans, the primary plan pays first (applying your deductible, copay, and coinsurance). Then the secondary plan may cover some or all of the remaining balance. In some cases, your out-of-pocket cost can drop to $0.
Is dual coverage worth it? Not always. You're paying two premiums. If the secondary plan's premium exceeds what it saves you in out-of-pocket costs, it's not worth keeping. Run the math: total premiums for both plans vs. total premiums for one plan plus expected out-of-pocket costs.
Birthday Rule for Children
When a child is covered by both parents' plans, the birthday rule applies: the parent whose birthday falls earlier in the calendar year has the primary plan for the child. This has nothing to do with age — just which birthday comes first in January-December order.
Related Terms
Last updated: March 30, 2026.
