Maternity Sash - possible idea. Though instead of a girly sash, have it like a vine/leafy sash to be gender neutral. Maybe design a sash and hold it over, then use it as a prop when the little dud/ette pops out?
At first I thought, "Oh that's cute" but then I thought, "A good big brother would later use this photo to convince his little brother/sister they were adopted/alien/left on the family's doorstep and taken in as charity." Could totally use this for callie