The comments on the site say the dress runs small. I found, on getting a large bump band from them, that the size ran large.
If you only have a couple of weeks till your wedding, you could call them and ask if they could send both sizes and you could return the dress you didn't want. That way, you would be sure of getting the best fit and more likely to get the dress you want in time, hopefully.
Congrats on both the bump and the wedding by the way!