You're going to grow, but maternity is made to grow with you. I'd go ahead and get one, I'd be shocked if it didn't fit. You could always get a maxi dress, that'll give you extra coverage.
My shower was at 24w and I can still wear the dress that I wore to it, with room to grow some more. Unless you're getting something more fitted you should be fine!
Re: Shower dress: How much would I grow in 3 weeks?