Maybe a empire waist dress. Or go to Davids bridal and tell your situation to the sales person and let them help you. I know they have had this situation before because my husbands cousin was a bridesmaid in her sisters wedding when she was 8 months pregnant. She wore a dress that matched all the others but had been altered to fit her bulge. I was strapless and was meant to be fitted at the waist. I suspect that it had been orginally way to big for her and basically remade to fit. I did look nice, but I think a empire waist dress would have made a better fit. I am attaching a link for some, but you will need to make sure that what you choose has enough gathers to allow the room you will need.
http://www.dessy.com/bridesmaid-dresses
Chose Empire Waist from the Silhouette section down the side. I would not order one online though. I would go and find one at Davids Bridal and ask a sales person about options or for help finding one. They may can special order you one as well.