Imagine Derek catching Stiles with his hand in the cookie jar, grabbing a handful of cookies with even more in his mouth. Stiles freezes like an animal in headlights and Derek scolds him. Lacking an excuse, Stiles shouts, “I’m pregnant,” and runs off with the jar.
Any other day, Derek wouldn’t have given it a second thought, but taking into consideration a full moon has just passed and ‘conveniently’ was in time with Derek’s heat, Derek starts to worry that it might be true.
In classic Stiles form, it’s not; it was just the first response he had, but because he couldn’t think of anything better he has to put up with a very panicked Derek who’s worried he’s not going to be a good father, crying about how he wants kids but not so soon, etc. And soon those cookies disappear from Derek’s stress eating.