Wedding in Sept, Due in Dec

Javonda β€’ I am a 26yo wife and mother to 3 daughters (2 stepdaughters and 1 daughter from prior re). Expecting our first son December '16 πŸ˜β€πŸ’™πŸ’œ
I am in my best friend's wedding 9/4 and I'm the matron of honor. I'm so thrilled because she was my maid of honor two years ago. I'm almost 5 months (17w1d) and I have to order my dress like tomorrow if I want it in time for alterations. My issue is I've gained 20 lbs already. My dr said I was underweight so I need 40 lbs at least during my entire pregnancy. I'm worried about my size at the wedding. I'm going to get fitted but I don't know if I should order a size or two up or what? Anybody been pregnant in a wedding?