A human baby not fully developed, but is carried to full term. A serious and lethal deformation. It may resemble a mass of cells or tissue, with or without hair and teeth.
The mole birth looks like a cluster of grapes. or
You won't be having a baby brother, unfortunately it's a mole birth.
In the Pilot Episode of Firefly (aired last by the same backbirths that cancelled it after less than a season) the "Purplebelly" Federal Agent sometimes called the "Mole" says, “Do you think I am a complete back birth?”