I would get the first week or so back at work over with before telling them - get yourself back into the swing of things first, and then you can cushion the news a little bit by moving swiftly on to talking about what you've completed/achieved so far in your week back! I wouldn't leave it until after Christmas, just in case people start speculating about a bump - better to tell your bosses before they overhear gossip I think.
I'm returning to work shortly, by which point I'll be 15 weeks pregnant (oops), and am in crisis about when to tell my employer. Should I get it over and done with now? Or wait a bit - say til after Christmas? (I can probably hide the bump as I'm not in the office that much.) Which would annoy them less do you think?