when to tell at work

Been quiet here for a while, but I'm jumping in...
I told my boss about my pregnancy after the first ultrasound during my last pregnancy. That was around 9 weeks. Their first reaction was "And was it planned?" I found that a bit strange, but apparently, that question is asked more often haha. I told my colleagues at 12 weeks. I just put a bowl of sweets shaped like pacifiers on the table with a cute note.
Now I'm pregnant with my second child, but since I teach (in Belgium), I won't see my colleagues and the management again until the last week of August. I'll be 12 weeks pregnant then, so I'll tell them a bit later than last time.

With 10 weeks

At 9.5 weeks... that was at the end of the summer holidays and I was standing in front of the class as a teacher. Given the nausea, I wanted to tell them. Then you don't get weird faces when you run out of the class
I had already had 2 ultrasounds... at the intake at 7+2 and with the dating scan at 9+2. And just before that, I had told my immediate family first.
My reasoning was also... if something goes wrong, they know it too... Then they also understand why I'm sad or not feeling comfortable in my own skin.
You are allowed to share in my joy and sadness is just as much a part of life. So I just shared it and didn't wait for some made-up 12-week deadline
Those were my reasons.... and everyone should definitely make their own decision.