when to tell work?

B
I found out I was pregnant 2 weeks after signing my new contract with a new job.
I've decided to wait a bit till I tell them as I lost three last year, 1 at 8 weeks and 2 at very early stages. 
I'm now 9w5d pregnant and I don't know when is the right time to make it public as I'm worried about work.
Any advice?