I attended a gala this past fall at 6 months pregnant. I didn't purchase any fancy maternity dresses but was able to improvise with this old non-maternity dress I bought at Express in 2007. That's practically vintage. Somehow, it still fit me if I tied the belt at the empire waist.
The sleeves were a bit tight so I wore them unbuttoned and put a jacket over the dress.
Dress: Express (from 2007)
Leggings: Motherhood Maternity