When Dee is set to give birth, Dennis gets stuck as the hospital taking care of her, while Mac, Charlie and Frank continue the mission to learn who the father is to bring him to the hospital.  The boys invited every ex of Dee's under the guise of a party.  When they can't figure out who it is, they bring everyone to the hospital where Dee reveals she was actually just being a surrogate for the Tranny.  Well played Always Sunny.