Theres no reason why you shouldn't go! Go! Have fun! Its y.O.U.R. Prom so enjoy it. As far as maternity dresses, I don't really know. I had a preg friend who went to our formal (same as prom sorta--really fancy thing) and she was 6 mnths pregs, she just bought a normal prom dress, only got a bigger size. She was a size 6 prepreg and I think she got a 8 or 10 for formal. Then she had a few alterations to make a nice fit on her belly, but nothing major, her mom did it (and she's no seamstress)
but check out on the web too--check ebay too! Thats where I got my prom dress--for 30bucks and it was reg. Over 300...People only wear those kind of fancy dressed like once, so you don't have to worry about them being all worn out lol
---either way, go to ur prom, even if you just wear normal clothes, go! Or you'll regret it forever!!!---
best of luck xxx