Help!

Hi ladies,
I recently found out I'm pregnant and I'm about 8 weeks along. I'm in my best friend's wedding in May (I'll be about 20 weeks then) but I had already ordered my bridesmaid dress before I found out I was pregnant. It's a long, chiffon gown that has a sweetheart neckline and really float from the bust down. Im hoping I can keep the dress I ordered since I have no idea what size I'll be by then! Has anyone else had to deal with this issue?!