SERIOUSLY don't worry about it too much! I have gotten away with ****tail (which your dress sounds like) to black tie before (2 events in one night) - just put your hair in an updo, nice jewellery, makeup, and some heels if possible. A small clutch will help the look too.

BUT more importantly - people are SO understanding when you are pregnant, I would never ever criticize a pregnant woman for being inappropriately dressed, it's so hard! Everyone knows that!
x
I was going to say exactly this. Just wear what you've got.

What are they going to do? Chuck you out because your pregnant?

Good luck for your husband on his award!!!