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.
"Resentment destroys more alcoholics than anything else. From it stems the root of our spiritual disease."
"In dealing with resentments, we set them on paper - list people, institutions and principles with whom you are angry. Ask yourself why you are angry."
"In most cases it was found out that our self-esteem, our pocketbooks, our ambitions, our personal relationships, including sex, were hurt or threatened. We were sore, burnt-up." Alcoholics Anonymous, 64-65.