Yep.
But it won't matter as much...they will bill incrementally...
My insurance was billed and paid out monthly, even thought I paid my entire annnual portion in a lump sum payment at the beginning of each year.

Yeah, because it's not a "one time" thing. It's not like surgery that was performed on day one and insurance started day 15. It's a rolling thing.
One of my kids had braces, insurance paid, then I left that job and started another, and that insurance paid their portion... it didn't matter that the kid already had them on.
Have you checked and made sure that the insurance covers braces?

Anyway, try not to worry right now. You, or DH, can call the insurance company tomorrow and ask them about it. Chances are, you're fine.

Well, if it helps..years ago when my ss had braces, my dh lost his job. He got a new job a couple weeks later (better times!) his new insurance went into effect 60 days after. The new insurance paid the remainder even though the braces were already on. Like stickafork said above, they bill in increments, not the entire benefit at one time.

BUT,,,,the right thing to do would have been to wait. Back in the day I always asked BM to please at least let me know when their dental appts were so that I could budget accordingly. Of course she never did. Needless to say, my skids went to the dentist every six months, like clock work. She never cared that their daddy was paying out the portion not covered. But then we did get the most expensive Christmas list items too!

At 16, I wouldn't expect a kid to understand how insurance works. Not unless someone told her. Why did Dad not tell her that it may not cover anything done before it is in effect? That's kind of a no-brainer, IMO...