I agree, that at 16 weeks, your bump probably won't be very big at all, and depending on your dress style, you will probably get away with it, perhaps just wait to get it altered until a couple weeks before the big day - if possible!
You would have had your 12 week scan by then, who knows, you may be happy to announce the fab news on the big day! Take each day as it comes, and try not to let the last minute wedding bits and bobs stress you out, and just keep rested (if you are anything like me you spend more time asleep than awake during the first trimester)!!
Good luck with your pregnancy and wedding (and the dress)!