When Mycroft kidnaps Sherlock to discuss something important (at least in Mycroft's opinion), Sherlock goes into labour in his car and gives birth before they manage to get into a hospital (or to get John). He's been having contractions for quite a while, but trying to hide it as best as he can to avoid showing Mycroft any weakness. But then the hell (and the water) breaks loose and it becomes too painful and too obvious to hide. And, yep, he gives birth. Mycroft is assured that Sherlock has done it just to spite him and is not amused.