I wore a black sweater to a christmas party and no one knew I was pregnant until someone asked me what I do and I told them nothing right now cause I'm getting ready for the baby. I'm 37 weeks. So funny how no one noticed! I would think a black dress and something loose on top would be your best bet.
Black is always slimming. I wore a dress for my holiday party with a loose sparkling top portion and the bottom was fitted but it was black and you would never know I was pregnant in that dress.
Cardigan or a pretty loose sweater with a wide v neck to draw attention away from the belly. I'm sure you look great and I was the same way. Good luck....