New Parents Hit With Huge Surprise Medical Bill Because Of The 'Birthday Rule' : Shots - Health News : NPR

... When a child is born into a family in which both parents have insurance through their jobs, the parents are supposed to "coordinate benefits'' — meaning they must tell both insurers that their child is eligible for coverage under two plans. The parents might be forgiven for thinking they have some say in how their child will be insured. 

In most cases, they don't. Instead, a child with double health insurance eligibility must take as primary coverage the plan of the parent whose birthday comes first in the calendar year; the other parent's insurance is considered secondary. This model regulation was set by the National Association of Insurance Commissioners and adopted by most states, including Kansas, says Lee Modesitt, director of government affairs with the Kansas Insurance Department...

https://www.npr.org/sections/health-shots/2021/01/27/961196647/birthday-rule-blindsides-first-time-parents-with-a-mammoth-medical-bill