I ended telling work when I was 10wks ... which was earlier than I had intended (since I'd only been working there for 6 wks at that stage), but my bump was starting to show and I didn't want to get any awkward questions.
As it turns out, they were absolutely fabulous about it, even though it must have been quite inconvenient, and my team leader ended up telling me that she was also pregnant (due a few days before me).