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.
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.