How soon did you tell your job you were expecting?

I work in an office job - it’s not physical labor or dangerous where they would need to know right away. I won’t be telling anyone at least until the end of the first trimester, and naturally I plan on telling my family first before anyone else. I’m an executive assistant, so it shouldn’t be hard to find someone to fill in for maternity leave, but I do want to be respectful and give them proper notice, time to prepare as needed. Right now I’m thinking around 18 weeks (this is my first pregnancy, so I’m assuming that will probably be around when I won’t be able to hide it for much longer) but wanted to get some opinions and see what the norm was. Thanks!