A Bun in the Oven

Yes folks, I'm pregnant. But isn't it much more fun to say, "I've got a bun in the oven"? And to refer to the little one inside of me as a "the bun", rather than "the baby"? But it's also a little strange, don't you think? Where did anyone ever come up with the term "bun"? Why would someone want to go to all the trouble to make just one bun, rather than a dozen? Why not a cake, or a pie? I guess it wouldn't sound right to say "I've got a pie in the oven" because that's actually believable... if you make pies. But no one would believe any sane person would make the dough, knead it, let it rise, etc. for just one bun. It would have to be one cute and cuddly bun.