When to tell your boss?

Hannah • Albie, 4/4/17 💙✨
Hello ladies,
I am in the UK and was just wondering when I should tell my boss I am pregnant? So he can carry out a risk assessment. I was going to wait until after my 12 week scan so that I know everything is ok but should it be before this? I am currently 5 weeks 5 days 🙂