When to tell boss????

I'm 6w2d pregnant with my rainbow baby #2 and I work as a custodian at a elementary school. We work with tons of chemicals and lots of lifting and on our feet from start to finish!:( I don't know when to tell my boss because I don't want them to let me go because of it I'm a 12 month employee which means at the end of my 12 months they will evaluate me and decide whether or not they want to hire me full time or not ( currently working part time ) my fear is that if I tell them I'm pregnant they won't want to hire me on full time due to risk with all the chemicals and heavy lifting:( my husband is an EMT and makes decent money but I don't think we can live off of just one income especially now that we are expecting a new little :/ I'm so stressed out !! I even had to have an ekg done because of the loads of stress are starting to give me severe chest pain:( I just don't know what to do