D.W.
Try a dress with an empire waistline (i.e., the waist just below the bra) and a flowing skirt. I have a maternity dress like this and it also has a tie in the back that allows the dress to adjust as you do. I'd let you borrow it, but it's got a flowery print, so it's more of a summer dress than a semi-formal one (and too cold for February).
Kid-To-Kid (on Eldorado and Lake Forest in the Kroger shopping center) sells gently used clothing for kids and maternity clothes also. You might be able to find something inexpensive in there that you can wear to both (or maybe even two inexpensive things, one for each occasion).