Very physical job, when should I tell work

Kaylee
Hi all I am just writing for some advice
I live in the UK and I work with learning disabilities. This involves challenging behaviour. As part of my role I am a moving and handling instructor and complete moving and handling of people on a daily basis, I'm on the trampoline (rebound therapy) and in the hydrotherapy pool nearly everyday!!!! 
Rebound and hydro are not advised when your pregnant however I am only 7 weeks and feel that is too early to tell work just yet! I'd feel better telling them after my 1st scan (between 10-14weeks). What do you think? 
I want to do what's best for me and my baby but if I told work now and something happened to my baby before 12 weeks I couldn't deal with that! I'd only tell my line manager and ask for the upmost confidentiality so that no other employees knew.