Hi Gidget
You have some fun in front of you - as yu deserve to pamper yourself and buy something new and glamorous! I think you will find that being a size 12 you will still have a lot of options - just play up the bits you do like...
I am also worried about this after I give birth though so I hear your worries!

Allanah Hill would be a good place to start, she is one of my favourote designers and I went to her for bmaid dresses when I got married as all my bmaids had v diff bodies (ranging from model thin and no boobs to a size 14). all of the girls were happy there and the sales assistants are always extremely helpful. She also does lots that is 2 pieces but v glam so you can play up and down bits - dresses are often harder for people who are self conscious or where proportions have chgd (i have also bought stuff from her since being preg for weddings for this reason).
As Veronika Maine is already a fave that could also work well as her stuff is so structured that it will define you well if you pick the right outfit.

I say go treat yourself, push the boundaries a little and try things on that you are not sure of and you will find something lovely, buy some killer heels too that will lenghten you and make you feel glam. and maybe go to a makeup place and have a makeover - you will be a million dollars as you deserve!

ps underwear may also do all the trick, something smoothing underneath (a slip for example) will iron out any lumps and bumps and make any dress look better. Or Nancy Gantz stuff if you wanna suck it all in!