Your belly button is your very first scar, left over from the umbilical cord and marking the spot where the cord was attached before you were born. Another name for the belly button is the navel, which doctors call the umbilicus. Before birth the cord is a lifeline that connects a baby to the placenta, and a baby in the womb does not eat or breathe air, so food and air reach the baby through the umbilical cord. Blood rich in oxygen and nutrients flows through it from the placenta to the baby so the baby can grow. The cord is long and thin, about 55 to 60 centimeters, or 22 inches, and only 1 to 2 centimeters thick.
The cord is twisted like a rope, with the two arteries winding in a spiral around the vein, and the whole bundle is wrapped in a thin, tube-like sheath made by the amnion, the bag that holds the baby. Inside the covering the vessels are packed in Wharton's jelly, a soft, jelly-like cushion that works like a built-in shock absorber. The jelly cushions the cord's vessels against being squeezed, twisted or bent when the baby kicks and turns. It is a gel with a rich matrix that surrounds the two arteries and the vein and resists squeezing and twisting, so blood keeps flowing.
Blood moves in both directions along the cord, which also carries waste away from the baby. The two arteries carry oxygen-poor blood from the baby back to the placenta, where the waste is taken out of it, and the cleaned blood returns to the baby along the vein. In the womb the baby does not breathe air, so the placenta does the work of the lungs, and oxygen comes in through the cord. Three shortcuts help the circulation: the ductus venosus lets a large share of the blood from the cord skip the liver and flow straight toward the heart, the foramen ovale, a small opening in the heart, lets blood skip the lungs, and the ductus arteriosus, a short blood vessel, sends blood past the lungs.
At birth the first breaths fill the lungs with air, and blood then starts to flow to the lungs instead of taking fetal shortcuts, a change that begins within seconds. After a baby is born, the baby breathes air with its lungs, so the cord is not needed for oxygen anymore. The sudden drop in right atrial pressure pushes the septum primum against the septum secundum, closing the foramen ovale, which usually seals within about 3 months and leaves a dent called the fossa ovalis. Breathing makes the muscle of the ductus arteriosus tighten for good within 10 to 15 hours, and it becomes the ligamentum arteriosum, while the ductus venosus closes and turns into the ligamentum venosum.
After birth the cord's blood vessels inside the body close up, and the belly button holds the sealed-off ends of the one vein and two arteries. The umbilical vein shrinks into a tough fibrous string called the round ligament of the liver, which stays there for life as a leftover of the cord, and the two umbilical arteries shrink to fibrous strings too, with their far ends becoming the medial umbilical ligaments, which stay in the belly for life, while the near ends stay as working arteries.
Right after birth, the cord is clamped, which means pinched shut, and then cut. Before the 1960s it was not unusual to wait five minutes or more to cut the cord, and then, for reasons that are unclear, doctors began clamping and cutting almost right away. The value of that quick cut was never shown, according to one NIH specialist, and doctors now study timing again.
As of November 2026, the group of obstetricians in the United States called ACOG advises waiting at least 30 to 60 seconds for healthy babies, in guidance from 2016 and later. Waiting lets extra blood move from the placenta to a full-term baby, giving the baby more iron stored in the body in the first months of life, and it supports the baby's first breaths. For babies born too early, waiting has been linked to a lower risk of needing blood transfusions, of anemia, and of bleeding in the brain.
Waiting may slightly raise the chance of newborn jaundice, a yellow tint to the skin that doctors watch for and can treat, and hospitals that delay clamping are set up to monitor for it. Doctors do not wait when the baby needs help right away, for example if the baby has trouble breathing and needs emergency care.
What the cut leaves behind is a short piece of cord, the stump, on the baby's belly. At first the stump is soft, moist and bluish white or yellowish green, and then it turns yellow, then brown or black, a bit like a scab, and dry, dark and crusty is normal for a stump that is healing. The stump dries up and falls off, usually within one to three weeks after birth, though some take a bit longer, and it leaves behind the dent we call a navel. The stump will come off by itself, even if it looks like it is barely hanging on.
Grown-ups are told not to pull or tug at the stump, and they do not put rubbing alcohol, lotion or anything else on it. If it gets dirty, they rinse it with plain water and a soft cloth, then pat it dry. A few drops of blood on the diaper around the time the stump falls off are normal, but if the cord actively bleeds, grown-ups call the baby's doctor right away.
Signs that a stump may be infected include a foul smell, yellowish fluid, red skin around the base and crying when the area is touched. Infections are rare, but a doctor should see the baby quickly, and a kid who sees a baby's stump that is red, swollen, smelly or bleeding should tell a grown-up. A baby whose stump has not fallen off by about 3 weeks of age should also see a doctor. A small, moist, pink or red lump of extra scar tissue, called an umbilical granuloma, can form after the stump falls off.
Many babies are born with an umbilical hernia, a bulge at the belly button that forms when part of the intestine pushes through a weak spot in the belly muscles near the belly button. Most umbilical hernias in children go away on their own, usually by about age 5, and a baby's hernia most often heals in the first 12 to 18 months of life. A child's hernia usually needs treatment only if it is very big or does not go away, and one larger than about 2 centimeters is more likely to need surgery. Taping a coin over a belly button does not fix a hernia or change an outie, and doctors say this old folk idea has no medical benefit and can cause a skin rash.
Most mammals have belly buttons, but you may not see them: on a dog or a cat the navel is a small flat scar hidden by fur, and chimpanzees and gorillas have belly buttons that look a lot like ours. Dogs, elephants, lions, bears and many other mammals are placental mammals like people. A marsupial, such as a kangaroo, is born very early and finishes growing in its mother's pouch, where a baby kangaroo is called a joey. Monotremes, such as the platypus and echidna, are mammals that lay eggs, and their young hatch from eggs. Platypuses have no umbilical cord and no belly button.
Doctors have 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. In 1958 a team in Glasgow, Scotland, led by doctor Ian Donald, published a paper in The Lancet that began the use of ultrasound to look at unborn babies, and its prototype scanner was made from a disused flaw detector, bits of Meccano toy parts and a hospital bed table. Scientists have also studied the navel as a home for bacteria: a study that used swabs from 60 volunteers found 2,368 kinds of bacteria in their belly buttons, with about 67 kinds on average in each one, though some had as few as 29 and some as many as 107. Only about 6 to 8 kinds were common to most people in the study, and most kinds were rare, so scientists compared belly buttons to rainforests.
Doctors use the navel as a reference point on the belly, for example when examining a patient, and they describe it with four parts: a hump in the middle called the mamelon, a dense scar called the cicatrix, a raised skin edge called the cushion, and folds called furrows. An innie sinks in, with the skin around it folding inward, while an outie sticks out, often because of a little extra scar tissue. Most belly buttons are innies, and outies are less common. No one knows exactly why some belly buttons are outies, though it may depend on how the cord area heals and on how much space lies between the skin and the belly wall. Some outies are caused by an umbilical hernia or an umbilical granuloma, and both are generally harmless, but a doctor should look at them. Whatever its shape, your belly button is your very first scar, left behind by the cord that once connected you to the placenta.
Words to know
- umbilicus: the navel
- ligamentum: tough string
- granuloma: extra scar lump
- monotremes: egg-laying mammals
- cicatrix: dense scar
