I got this idea from another online forum and used it with my firstborn on Labor Day weekend last year.
Our family gets together every year at our house. When the time came for the big picture, I said, OK, I'm going to take 2 pictures, so stay where you are:
Then I said, "OK, say CHEESE!" for the first one.,
Then, for the 2nd one, I said, "OK, say: WE'RE HAVING A BABY!".
It was hilarious. Family members couldn't figure out why I said that. Then, some of them "got it". I also rubbed my belly and said, "Yup, we're 11 weeks along..". Then, everyone ran around hugging, kissing, screaming and crying. It was better than winning the lottery.
My sister also had her first-born wear a t-shirt at a previous get together that read at "Baby brother or sister in March 2003". That was really cute too.