If the oldest child is old enough to understand the whole pregnancy thing and she is a talkative person, expect her to blurt to everyone you meet, Mom's pregnant!
We went to eat at a pizza buffet on school night (kid's school gets part of the money from purchases), we saw many people from the school. It was somewhat embarrassing to have the daughter announcing it to everyone who walked past.
I'm taking my time in announcing things especially at the office. They'll have to wait until after my appointment on October 25. It ain't like I'll be showing anytime soon. Plenty of time to share the news. The only drawback about not sharing in the first trimester is that's the time we're feeling like cr@p and it isn't easy to hide.