If the fetus wants to grow and develop in the mother's womb, it must constantly absorb nutrition and oxygen from the mother. However, in the mother's womb, the fetus can't eat with its mouth and breathe with its nose. Everything that a new life needs in the process of gestation can only be absorbed by the placenta and delivered to the fetus through the umbilical cord. After the baby was born, the placenta and umbilical cord were gone. The original functions completed the historical mission, so the doctor cut them out of the baby. Because there is no pain nerve on the umbilical cord, the baby will not feel pain. The remaining piece will automatically fall off in a few days, leaving a small belly button on people forever.

At a certain stage of the embryo, the umbilical cord is a "gateway" extending in all directions, which is connected with both the bladder and the yang. With the development of the fetus, these connected parts gradually degenerate and separate, and the umbilical cord is "cut off" from the intestine and bladder. If the degeneration is incomplete for some reason, urine and feces may leak from the navel. But this kind of situation is rare after all.