When to tell @ work?

I'm 10 weeks and we're planning on telling family and friends in the next couple of weeks after we get back all our tests. When should I tell work? There are 2 girls at work that it was soooo obvious that they were pregnant, but didn't officially confirm the news until 15 and 20 weeks. I recently started a new position at work and was just confirmed for full time employment and don't want my boss to find out by the bump. So, when should i tell?