telling my class

So I teach 4th and I will be 14 weeks on Saturday. A few teachers know around school, as does my administration. I'm wondering if I should tell them on Friday before the long weekend. I'm thinking now because I am starting to show and would like to start wearing maternity clothes. The only concern I have is a coworker just lost her babies at 20 weeks and her 5th grade class was devastated. I know I can't wait forever and something can happen at any time, but when do you think it's appropriate to tell a class of 4th graders?