I think I'd find a tailor in town who can make the dress the week before the wedding. You could still be tiny, or you could be a lot bigger.
I've been a pregnant bridesmaid, and had a pregnant bridesmaid in my wedding. We used a different dress pattern, but similar neckline and sleeves and the same fabric. Some bridesmaids dresses don't do well as maternity dresses.
You have 20 weeks to get this sorted out - don't just let her make a standard fit dress several sizes too big - that won't work.
Honestly it all depends on the person . Some people gain a little some gain a lot some gain just belly everyone else gets it everywhere (butt boobs legs) my friend was just in a wedding and before she got pregnant she was tiny and they tryd to order it bigger but she only gained belly and it wouldn't zip she had to take it to get resized 2 weeks before the wedding. I would honestly just get it a few sizes up and if it don't fit by the wedding resize it
I was just a bridesmaid. They ordered my dress 3 dress sizes bigger. 29 weeks when the wedding came and the dress came back huge. But it's easier to take in a dress then to let it out.
Wow, that's a hard one. I'm 30wks and only gained 12lbs and I am absolutely huge, all belly. So hard to know how much you will gain and how big your belly will get. I would be leaving your dress til last to make and see how you go.