Why do some people have innies and some have outies?

Many mothers ask me if their babies will have an “innie” or an “outie”. I can tell by how much skin grew out from the baby's body toward the umbilical cord. As the cord falls off, the baby is left with extra skin hanging from their “outie” belly button. Babies whose skin doesn't grow out toward the umbilical cord are left with very little extra skin on their bellybutton, an “innie.” Do you have an “innie” or an “outie”?