The umbilicus, commonly known as the belly button, is a small, depressed or protruding scar located at the center of the abdomen. This feature marks the spot where the umbilical cord once connected a developing fetus to the placenta during gestation. Curiosity often centers on whether the umbilicus is a continuous passageway or a fully sealed structure. Understanding how the body closes this former connection after birth explains why it transforms into the fibrous remnant seen in adults.
How the Umbilicus Forms After Birth
The umbilical cord serves as a lifeline in the womb, providing oxygen and nutrients to the fetus. At birth, the cord is clamped and cut, leaving a small stump attached to the newborn’s abdomen. This remaining portion is composed of connective tissue and the remnants of blood vessels.
Over the next one to three weeks, this stump naturally begins a process of drying and shriveling. It changes color, typically darkening from yellowish-green to brown or black, before detaching on its own. This separation is a normal part of the healing process and is painless for the baby.
The resulting mark on the abdomen is the umbilicus, which is fundamentally a scar. Its final appearance, whether a concave “innie” or a protruding “outie,” is determined by how the skin heals and folds around the fibrous tissue left behind. The specific shape is largely a matter of natural variation and how the underlying abdominal wall closes.
Is the Belly Button a True Opening?
The belly button is not a true hole or an open passage that leads directly into the abdominal cavity in a healthy adult. It is a fully sealed and structurally sound structure. The skin of the abdomen folds inward at this point, creating the visible depression or protrusion.
Beneath the skin, the former opening, known as the umbilical ring, has closed. It is reinforced by dense layers of connective tissue and muscle. This strong, fibrous scar tissue, known as fascia, seals the gap where the umbilical cord once passed through the abdominal wall.
The closure of the abdominal wall ensures that the area remains protective, acting like the rest of the skin on the body. While the umbilicus represents the weakest point in the anterior abdominal wall, the combination of skin and underlying fibrous tissue creates a complete, non-patent seal in the vast majority of people.
Internal Structures: What the Umbilicus Used to Connect
The closure of the belly button involves more than just the external scar; it also includes the internal transformation of the structures that ran through the cord. During fetal life, the umbilical cord contained two umbilical arteries and one umbilical vein. These vessels were the main conduits for blood flow between the fetus and the placenta.
After birth, when the cord is cut and the newborn begins to breathe independently, blood flow through these vessels ceases. This change triggers a natural process where the vessels atrophy and close off inside the body. The now-closed umbilical vein becomes a fibrous cord known as the ligamentum teres, or the round ligament of the liver. The two umbilical arteries close and transform into the medial umbilical ligaments, which run toward the bladder. The urachus, which connected the fetal bladder to the cord, also closes to become the median umbilical ligament.