It's Always Sunny in Philadelphia Season 6 Episode 7
Who Got Dee Pregnant?
Dee tells the gang she's pregnant and one of them is the father from a Halloween party they can't remember. The guys bring in help from each other, Artemis, and the McPoyle twins to try and recount the night to figure out who the father is. They get it narrowed down to Dennis (gross), but Dee reveals it's none of them.