when do you tell work your pregnant?

I recently found out I'm pregnant, but I am yet to see a midwife I'm due to go on Thursday, iv worked out I'm around 5 weeks 6 days, I work in a care home for residents with advance dementia and a use a lot of hoists,moving and handling as well as having to be weary that I'm not nocked or bumped by the residents as they can become quiet irate, upset, when's the best time to tell my manager? will other staff then have to know I'm pregnant also? do I need anything from my midwife to show to my employer?