Bridesmaid dress 1-3 weeks postpartum?

Alycia
Has anyone had to order a bridesmaid dress 1-3 weeks postpartum? My sister is getting married Sept 30 and I'm the matron of honor, due sept 15 :-/ I obviously have no idea when he or she will actually come, and I have no idea what size I'll be. I am typically a 4 or 6 before pregnancy (I'm in another wedding in July and already ordered a 10/12) but the immediate post partum period makes me nervous. Any recommendations? I have to order it online this week.