Not gonna lie- I was pretty proud of myself for coming up with this (and all I needed was a fat Sharpie and white T-shirt. I then 'made' the costume while sitting in the car in the driveway of the house the party was at). First person to figure out what I was wins (you don't actually win anything- but you will have the satisfaction of winning. How's that for taking you back to 'things you heard when you were a kid'?).
*hint: It does have something to do with pregnancy