We announced both of our pregnancies early, as soon as we tested positive to friends and family and Facebook announcements with the 8 week ultrasound. If you have no history of miscarriage, I seen reason to wait. We started talking about the baby in my belly immediately with our daughter, then only 16 months and by now she knows that her baby brother is in there and she talks to him, kisses him and cutest of all plays peek-a-boo with him through my belly button. I'd tell your daughter soon, maybe bring her to some ultrasound appointments and make it a part of everyday conversation. I also recommend a book called "A Child Is Born" which has developmental photographs from conception to birth. I used it to help my daycare kids understand my first pregnancy and they loved it. Great way to help them wrap their minds around what can be a pretty abstract concept for kids. It helps them understand the time frame of the wait if you put it in terms of holidays when you're talking about it (ie. After Halloween, after Thanksgiving, after Christmas and Hanukkah, after New Years, after Valentine's Day, after St. Patrick's Day, THEN the baby will be born). Good luck and congratulations!