We're expecting our first child in October and my wife and I have different health insurance plans

Yes, it’s possible that your wife's plan may have to serve as the primary carrier – if your wife's birthday falls before yours in the calendar year.

Sometimes parents want to have their children covered by more than one health insurance plan to maximize health benefits. In that case, the plans must coordinate the benefits between each other to make sure health care providers are not reimbursed for more than they're entitled. A common way to coordinate benefits from more than one health plan is through something called the "birthday rule."

Under this informal rule, the health insurance plan of the parent whose birthday comes first in the calendar year serves as the primary carrier, and the health plan of the other parent is considered secondary. It doesn't matter who's older because the year of birth doesn't come into play -- only the month and day.

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The primary plan pays the health claims first, and the secondary plan pays any remaining costs not covered by the primary plan. Beware, though, that the secondary plan kicks in only if the medical care is a covered benefit under that plan.

Keep in mind, this is an informal rule and not a law. Also, there are some exceptions to it, which can get complicated. To learn more, read how the “birthday rule” determines health insurance coverage and check with your health plans to get additional details.