Bridesmaids dress sizing 4 weeks after giving birth

I'm in a wedding 4 weeks after my due date. We have picked a dress and t has an empire waist and is very loose and flowy. I'm five months pregnant now. They told me I should go a size or two up from the size I am now. I know I will still have plenty of weight on me after giving birth but a size or two bigger than five months pregnant seems like a lot. Any suggestions? I have no clue what size I will be 4 weeks after the baby (or less if I go past my date). Help!! Attached is a picture of the dress.