Bridesmaid dress advice

Not sure where to post....

I need some advice on purchasing a bridesmaid dress. I’m currently 7 months pregnant (due end of September), the wedding is in January. They picked out a bridesmaid dress (from David’s bridal) but I have no idea what size to order. I don’t even know what size I was before I got pregnant. I obviously would like to lose weight after baby is born/before the wedding, but I’m worried I’m going to gain weight after like I did after my first. I’m guessing I’ll be either a 14 or 16, but really have no clue. Any advice? I’m being pressured into buying it sooner than later, and i think I’ll only have 14 days to return it after receiving it. So what do I do/what size do I order??

Sorry for blabbering on, I just have a lot of anxiety about it. TIA!