When do you tell your boss?

I am TERRIFIED to tell my boss, I’m only 5 weeks but my kids are at my work before school and I’m so afraid they’re going to spill it. I’ve only been there 3 months and am officially receiving my contracted position next week.... my first appt is March 4- should I say nothing and let him notice? Tell him after my first appt? Or after my first trimester? What would you say if the kids end up spilling it first???