To prevent this, insurance companies typically designate one parent's health insurance plan as the primary plan and the other as the secondary plan. (That's why the patient questionnaire at your doctor's office asks for information on primary and secondary coverage.) The primary plan is responsible for paying covered expenses up to the limits of the policy. If any unpaid costs are left over, the secondary coverage kicks in. The birthday rule is often used to determine which plan is primary and which is secondary. Under this rule, the plan of the parent whose birthday occurs first in the calendar year is designated as primary. The date of birth is the determining factor, not the year. So it does not matter which spouse is older.

Like most rules, the birthday rule has exceptions: If both parents share the same birthday, the parent who has been covered by his or her plan longest provides the primary coverage for the children. If one spouse is currently employed and has health insurance through a current employer, and the other spouse has coverage through a former employer (e.g., through COBRA), the plan belonging to the currently employed spouse would be primary. In the event of divorce or separation, the plan of the parent with custody generally provides primary coverage. If the custodial parent remarries, the new spouse's coverage becomes secondary. Read your policy carefully to make sure you understand how your insurance company handles dual coverage.