I had a big age gap between my 2nd and 3rd too. I didn't tell my girls until I was past the 12 weeks. I would make excuses and everything, it was insane. I know that they picked up on things before then but I didn't confirm anything until then. After I told them, they pretty much announced it to the entire school population within a day, so I am glad that I kept it to myself for a while. It took my kids a few weeks to get their heads around the whole another baby in the house idea.....it was pretty tense for a while. But once she arrived they couldn't keep their hands off her!!