In medicine, anything having to do with the belly button or the cord that connects a newborn baby's navel to the placenta is umbilical.
One thing all prenatal mammals have in common is an umbilical cord attached to a placenta. It's how they receive nutrients and eliminate waste before they're born. Evidence of your umbilical cord is your navel, or belly button. Whenever you see the adjective umbilical, you know it either has to do with that cord, or figuratively, someone or something extremely close or connected: "Her relationship with her best friend is umbilical."