Make a list of factual information related to the baby’s current status and conception. You can include date of conception, baby’s approximate current weight, gender of the baby, due date and similar things. A list of 5 things should be plenty; more if you have a lot of guests. Get all your guests to place “bets” on each of the listed things; one bet per item per guest.
You can easily take the bets as people arrive to get things going and keep things lively while you wait for all the guests to show up. After everyone has arrived and placed their “bets” the mother to be gives away the answers and declares the winners. Award prizes to the winners based on who had the most and closest answers.