When to tell employer ?!

S

Hey! This is my first baby and I’m already stressing about work and being let go or replaced since I am a non tenured teacher . When is everyone planning on telling their employers ? I was thinking right after the first trimester is over but I’m not sure if I’m supposed to know exactly how long I’ll be on maternity leave got to tell my principal . I would love to be out at least a year but it will all be unpaid and I’m not sure if we can afford for me to be out a whole year . Any advice would be appreciated .