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I think a cocktail dress is perfect. Otherwise pick something simple in black and you'll be fine. Many years ago when I was pregnant, I had a dressy party to go to - I borrowed a dress from a friend - who wants to spend money on a dress you're not going to wear again? For my sister-in-laws wedding I was able to find a dress that wasn't a maternity dress, but was loose, and layered - it was perfect.