Maternity Evening Wear

I am going to a wedding in the Hamptons next month. The daughter of a family friend, someone I spent lots of vacations, summer weeks, etc. with, is getting married. It should be fairly fantastic and according to my mother, very fancy schmancy. My mother has informed me that even the rehearsal dinner will be black tie. Not a problem, right? My sister has fancy dresses galore and we are roughly the same size. Oh wait, I'm pregnant! Well damn. What do I wear?
I have done lots of window shopping online since I got the news, trying to find something and to get an idea of what sort of cost I'm looking at. I kind of hate to spend big bucks on a one time deal, you know? My sister and I and a friend have set up a girls shopping trip to try to outfit ourselves for the wedding but that's still a few weeks away. Actually, it's a week before the wedding. I got to worrying that I wouldn't be able to find something that I liked, and then I'd be stuck looking like a dowdy matron. So I began shopping with a vengeance. And then yesterday I found something. On sale, no less. Originally $168 marked down to $42. I just pray it fits! As long as it does, I'm covered for the wedding. Now I just have to find something for the rehearsal dinner...