How long have the two/three of you been together? In a year or two, she might be sad you weren't there. Do you feel that she wants to nest with Eric and you have less of a role in their life as a result? Have you talked about this co-parenting thing?
I know it does nothing to alleviate this particular sting, but this might not be her only birthing. It might be that one day she will there to hold your hand when you give birth.
Me: bi female in my twenties