Look closely at your navel and you are looking at what remains of the umbilical cord, which at full term measures about 55 to 60 centimeters in length. The cord is a lifeline that connected you to the placenta. Food and air reached you through the cord, because a baby in the womb does not eat or breathe air in the way that a newborn does. Blood rich in oxygen and nutrients flowed along it from the placenta so that you could grow. When the cord was cut after birth, it left behind a scar, and that scar is your belly button.

Inside the cord run three blood vessels, one vein and two arteries, and a newborn's cord is only about 1 to 2 centimeters thick. The arteries wind around the vein in a spiral, which makes the cord look twisted like a rope. The vessels sit in Wharton's jelly, which works like a built-in shock absorber. It cushions them against being squeezed, twisted or bent whenever the baby kicks and turns. Doctors have also studied the cord by slicing it very thin and looking under a microscope, which shows the cord's two arteries, one vein and the Wharton's jelly that fills the space around them.

Traffic in the cord moves in both directions. Blood rich in oxygen and nutrients flows from the placenta to the baby. The two arteries carry oxygen-poor blood back to the placenta, which takes the waste out of it so that the cleaned blood returns to the baby along the vein. Since the baby does not breathe air in the womb, the placenta does the work that the lungs will one day do, and oxygen comes in through the cord. The cord is a lifeline that connects the baby to the placenta, and food and air reach the baby by way of it.

A baby in the womb has circulation shortcuts that a newborn will not need. A shortcut called the ductus venosus lets a large share of the blood from the cord skip the liver and flow straight toward the heart. Another is a small opening in the heart, the foramen ovale, which lets blood skip the lungs before birth. These shortcuts suit a baby who gets oxygen from the placenta instead of from air. Blood rich in oxygen and nutrients reaches the baby through the cord, so the baby can grow even before the first breath. The placenta does the work of the lungs, and oxygen comes in through the cord.

The switch-off begins with the first breaths, which fill the lungs with air so that blood starts to flow to the lungs instead of taking fetal shortcuts. The change begins within seconds. After the first breaths the foramen ovale closes, usually sealing within about 3 months, and it leaves a dent called the fossa ovalis. The baby now breathes air with its lungs, so the cord is no longer needed for oxygen. Inside the body, the cord's blood vessels close up as well, and the belly button holds the sealed-off ends of the one vein and two arteries. These remain as fibrous strings for the rest of life.

Doctors do not always cut the cord at once. In the United States, the group of obstetricians called ACOG advises waiting at least 30 to 60 seconds for healthy babies, as of the 2016 and later guidance. As of November 2026, that guidance stands. Waiting before cutting the cord lets extra blood move from the placenta to a full-term baby, and this gives the baby more iron stored in the body in the first months of life. Doctors do not wait to clamp the cord when the baby needs help right away, for example if the baby has trouble breathing and needs emergency care.

Once the cord is cut, a short stump stays on the baby's belly, which is the short piece of the cord that the cut leaves behind. It heals much like a scab, drying, shrinking and falling off, usually within one to three weeks. The belly button shows underneath, and the opening inside it seals itself shut. While the stump is healing, it should be kept clean and dry. A cord stump should be left alone to fall off by itself, and grown-ups are told not to pull or tug at it, even if it looks like it is barely hanging on.

Some signs mean a kid should speak up. Red skin around the base of the cord is a sign of infection. If a kid ever sees a baby's stump that is red, swollen, smelly or bleeding, the kid should tell a grown-up, and a doctor should check it. A few drops of blood around the time the stump falls off are normal, but a grown-up should call the baby's doctor if the area bleeds more than a few drops. If the stump has not fallen off by the time the baby is about 3 weeks old, the baby should see a doctor.

Why is one navel an innie and another an outie? Most are innies, in which the skin folds inward, while an outie sticks out, often because of a little extra scar tissue. No one knows exactly why some belly buttons are outies. It may depend on how the cord area heals and on how much space lies between the skin and the belly wall, which changes how much of the stump stays visible or tucks away. It is mostly a matter of chance, and nobody gets to choose. Taping a coin over a belly button does not change an outie. Doctors say this old folk idea has no medical benefit and can cause a skin rash.

Other mammals have navels too. Dogs, elephants, lions and bears are placental mammals like people, so they grow inside their mothers and get food through a placenta and a cord. A marsupial such as a kangaroo does have a placenta, but its young are born much sooner and spend much less time feeding from it, and they finish growing in their mother's pouch. Monotremes, such as the platypus and echidna, are mammals that lay eggs. Their young hatch from eggs, instead of growing inside their mothers. The platypus lays eggs, so it has no umbilical cord and no belly button.

Belly buttons have also been studied closely. In one study, scientists swabbed the belly buttons of 60 volunteers, counted the kinds they found, and reported 2,368 kinds of bacteria. The average belly button held about 67 kinds, though some held as few as 29 and some as many as 107. Only about 6 to 8 kinds were common to most people, and most kinds were rare, which is why scientists compared belly buttons to rainforests. So the spot where your cord was attached, the lifeline that fed you before birth, is now a scar that scientists have swabbed and counted. That scar is your belly button.

Words to know

• umbilical: of the cord
• Wharton's: cord cushion
• placenta: baby's supply organ
• marsupial: pouch mammal
• Monotremes: egg-laying mammals