Pregnant Best Woman...?

My fiance & I just found out that his "Best Woman" (best friend since childhood who happens to be female) is pregnant. Her due date is 3 weeks before the wedding. She has made it clear that she still wants to be in the wedding, so that's what we're planning on at this point. So the question is, how to we plan for the dress she wears? I had originally planned to get her a simple dress to match the groomsmen (black if they wear black tuxes) and then a sash in the color of the bridesmaid dresses to tie the look together. I would think she would wear a maternity dress if she was actually pregnant at the wedding, but she will have already given birth. Should we buy a dress that fits her while she's pregnant and then have it altered at the last minute? Is that even possible? It's not like we can predict how much weight she'll lose before the wedding, especially since she could deliver early or late. Any ideas would be wonderful, thanks :)