How early do you tell your BOSS you're pregnant?

Courtney
I don't know what I should do. I'm a 2nd grade teacher and school is just around the corner. My first appt is the week after school starts. I took an afternoon slot so I can just do a half day, but I feel bad that it's so early in the year- I know you other teachers can relate! Do you think I should go ahead and just tell my principal so she doesn't wonder why I'm already taking off on the 5th day of school??? Advice is greatly appreciated! I'll be 8 weeks along, FYI.