I told my boss at 9.5 weeks - IO was just too excited to keep it in!
My dad also works for the company - in a different area but he cou;dn't tell anyone until I told my boss - apparently he found that quite hard, so he was glad I "told early"

One of the girls here waited the full 13 weeks and another came out about 6 weeks. The 12 week mark is the safety mark going into trimester 2 and usually after the Downs Syndrome test.

All depends on how much of a secret you can keep