Teachers announcing pregnancy?

Fellow teachers, when are you planning on announcing your pregnancy to your co-workers and administration?  This is my first pregnancy, and I'm due April 1st. After the baby comes, I plan on using all of my sick days and taking the rest of the year off. I have my first US appointment at eleven weeks and thought I'd be okay to announce after that. Also, since I teach in a very small school, I'm sure my high school students will find out the same day I tell staff.  What are your thoughts or experiences?