Help! Telling my boss!

I am a teacher and I absolutely love my principal. This is only my first year working at this school. Due to not being tenured I can get laid off and they don't have to give me a reason. (They could say they are making cuts but maybe the truth really is due to me being pregnant.) Anyway, I was thinking of telling my principal soon because my "promotion" is being approved by the School Board possibly on Wednesday. I'm looking for some ideas on how to tell my principal. Do I just say,"I'm pregnant," or anything to lead up to it??? Any help would be appreciated, I'm very nervous and don't want to lose my job.