I'm a geologist and told work at 13 weeks.
Thank you! That's great advice. I'll have to start planning now. 12 weeks is next week!
I say wait till 12 weeks. When the chances of miscarriage are less likely just like every other case. When you go to doctor have them write you a letter stating your due date and a estimated time the doctor would want you out of work depending in your job duties and such. Meet with your supervisor tell them and give them the letter. So they have a amount of time to find a replacement or get someone in there that you can train.
Most people tell around 12 weeks, the end of the first trimester. However, if your job involves anything that would put the baby at risk, tell them now. Try to have a plan in place as well for when you plan to take your maternity leave. It will be easier if you can address their concerns right away, instead of saying, 'I don't know' to everything. Congratulations and good luck!