I don't have a pic but in my friends wedding the MOH was roughly 8 months pregnant with her first.
I remember she got the dress we got, then ordered a few yards (seemstress would know how much) of extra fabric and they re panneled her dress. Ours was a square neckline and then more a line vs empire but I believe they made her's an empire waist. (This was like 2002)
I remember it was a total PITA and her's cost almost double what mine did due the all the extra work (just an FYI), and her fittings were done like the week before the wedding to get correct sizing right.
I would take the advice of the seamstress unless it's not someone you "trust". You might want to get recs from people in your area about a good seamstress, just to make sure. I don't think she ordered bigger, she ordered her size and then they cut the front and added the extra fabric for her belly.