If you think he responds with something normal, like “Oh, congratulations!” or “Yay, more Hofstadters!” then you don’t know Sheldon.
Instead he just notes that it’s good Penny’s pregnant because she can’t make him sick, and this pisses Leonard off to no end.
We won the Nobel Prize, any idiot can have a baby.” Ouch!
And, it only gets worse, because when Leonard tells Sheldon that he should have more to say to a woman who’s loved and cared for him for 12 years, and that they were keeping it a secret so as not to upstage their big day, Sheldon replies with, “You couldn’t upstage us.