I told my boss when I was 8 weeks only because I'd just started working there just before I became pregnant and didn't know how she would react. I thought if I told her then and she wasn't happy then I'd still have enough time to get a new job. But she was fantastic and basically told me she thought I'd be pregnant in the near future as I was similar age to her and she had 3 kids. Anyway thats just what I did and my reason behind telling her so early.
I'd be telling your boss before anyone gets any idea and starts gossiping too