J.H.
I had this exact thing happen... and was able to exchange the dress at David's Bridal for a much larger size (originally ordered a size 6, exchanged for a size 12 or 14). I was 7 months pregnant at the time of the wedding. A fabulous seamstress was able to make alterations and the dress fit beautifully... it was a very simple strapless dress. It'll work out -- don't stress! Enjoy being pregnant and enjoy the wedding -- it'll be memorable!