The first baby born into a group of female friends. The use of the word "beta" implies that the arrival of this child will bring out a variety of friends' personal issues and test the strength of the bonds that hold the group together.
Friend #1: "Angie never has time to go out with us anymore. She was a lot more fun before she became a mom."
Friend #2: "Uh-oh, sounds like someone's resenting the beta baby."