Tinkle in the pot:
Very American, but fun. The idea is that you are heavily preggers and struggling to hold your bladder. Give each guest a balloon to be their 'bump', they can either pop this under their top or hold it in front of them, and a ping pong ball to put in between their knees (or tennis ball, satsuma etc) to be their 'pee'. Use buckets or plastic flower pots to be the 'toilet'. The guests need to go from one end of the garden or room to the other, and get their ping pong ball in the pot, whoever does it the quickest or the most times in a minute wins. You can also get people to team up and do it relat race style.
Get some plain white bibs or babygros and some coloured sharpies, have people decorate them and the mum to be chooses her favourite one as the winner.
Nappy changing race: either use cuddly toys or balloons to be the 'baby', if using balloons then you can squirt a bit of mustard of them first to make it a bit messier.