When I was pregnant with my 2nd child, I found out in late October. We decided to tell everyone at Christmastime when we celebrated.
I made up a card that said something to the effect of "Merry Christmas Grandma and Grandpa", and on the inside, included a note from Baby #2. Essentially the message said that "I am looking forward to meeting you on about August 15th", and was signed by Baby #2.
Since I wasn't showing, and no one had been told, it was a big surprise to all. [Hubby had a hard time not saying anything for about 2 months, tho].
When we presented the cards, we did it at the end of the Christmas celebration so it wouldn't "take away" from my daughter's Christmas. I wanted our announcement to be separate (if you will).
Just a thought...