You can't fake a pregnancy. I mean, let me explain...
If you alert the media you're having a baby, they expect to see it fall out of you sooner or later. If it turns out to be a complete wierd conspiracy-lie, then you'll never get credible work again (this goes for Tom Cruise, too), and the media will gnaw your wrist off.
Tom and Kate may live in a strange bubble, but they're not so disillusioned that they would do something that crazy. Hopefully.