Need advice... UK only

Jade
Ok so I'm currently 22 weeks pregnant I've been trying to find work for the past few months and I've finally got a job in which I start on the 21st November, I haven't told them I'm pregnant so how do I approach this? It's a temporary position which could possibly become permanent, but I'm worried if i tell them now they will find a excuse to get rid of me before I start. I will be 30 weeks when the job is due to end.