I use www.motherhood.com. They can be pricey, but if you look in the sale section they're pretty reasonable! Have fun at your party!

Did some hunting for you

http://www.duematernity.com/formaloccasion.html

I really loved the dresses on here

I shop at A Pea In A Pod, & Motherhood Maternity. Pea in a Pod is a bit pricey... but sometimes if you can find clearance deals, you can get some cute clothes. Motherhood had lots of great deals! And cute clothes. Hope you find something great for your party!! =)

Id definetely try motherhood maternity.
But you dont even need to wear a maternity dress. Anything that has an empire waist would be great to try and you could even buy right now. I would just shop around at the malls and try some stuff on. You won't be huge at 21 weeks yet but you will be noticably round. But I would shop around first before you spend money on a maternity dress I find them to be more expensive. Good luck (you look fantastic BTW )

I'd definately go with motherhood maternity, that's where i've been getting all of my maternity clothes from and they're cute and if you find something on clearance they're not bad at all, some things that even aren't on clearance are pretty reasonable, you also get your stuff very fast, I usually get everything in less than a week after I order it

I was looking at Target the other day and they had some really cute dresses both maternity and regular that would definitely work at 21 weeks. They were only like $40.

Also, I got some really cute work dresses at Motherhood Maternity. I went to their outlet and they had TONS of really cute fancy dresses.

I just had my husbands company Christmas party and I bought a regular dress just 2 sizes bigger to fit the boobs and the belly I already have. Everyone loved it, and i had a cute lil shaw. The good thing about the dress is that it dosnt make you look huge but if you want to show people your bump it is very easy to just put the bottom close to your skin. Oh and the best part was that it was VERY comfortable and nothing pressing on my bump. The dress was dark green with a black lace overlay.

http://www1.macys.com/catalog/produc...tegoryID=29272

I've found the empire waist dresses work great with pregnancy. They give you room to grow. At 21 weeks you could probably get away with just a normal dress from Macy's as long as it has empire waist =)