Right?!
It was obviously jewelry because the box was small.
I'm reminded of a Christmas fanfic that is actually in my rec list in which he gives her an heirloom necklace of his grandmother's for Christmas.
A Christmas Miracle by BUFFY_ANGEL2000
So I must wonder if that birthday present was a piece of family jewelry.
When he gave it to her he'd started to say:
"I'm a champion, we do important things and who's more important than--"
He, unfortunately, gets cut off by the rest of the gang shoving their gifts at her and then her coma but there is no way in hell the end of that sentence wasn't going to be "the mother of my child".
That indicates he put some real thought into that gift.