Primary/secondary insurance determination problems

Hello everyone, I'm trying to deal with a mess between Uhc and BCBSTN for the birth of my son. I had BCBSTN for a few years before my son's birth in May '21, it was not through marketplace (didn't qualify for any significant assistance), and after his birth it was proposed to me that I could be put on my employer's Uhc of Heritage Valley policy. I joined Uhc in early June '21, but had to pay premiums retroactively for Uhc policy to day of son's birth, making me have double coverage from birth to June 30, so like 2 months. So since then I've been having a nightmare now for 20 months as I keep having claims denied from these 2 months due to each insurance claiming they are not primary, etc. My question to anyone who can help us this: from birth of son until June 30, 2021, at which point my original BCBSTN policy expired and Uhc became my family's only insurance, who was my primary insurance and who was secondary? I was told initially that any employer policy will be primary, but the other day my wife's Obgyn (who is really getting the the end of the stick regarding all this billing!) told me that the policy that I had first and longest would be primary. Have to call both insurance companies on Monday and set things straight, but need your opinions because I know nothing about this stuff! HELP PLEASE!!! TL,DR: Had BCBSTN for years, son born May '21, got on employer's Uhc, had 2 policies for 2 months. Who is primary, who is secondary?

submitted by /u/Negative_Interview94
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