The umbilical cord forms early in pregnancy, within the first weeks. It grows from small stalks that connect the embryo to its supply, including the yolk stalk and the allantois, and later the growing amniotic sac wraps around them, so the cord gets its covering of amnion. At full term the cord is about 55 to 60 centimeters long, about 22 inches, which makes it a lifeline that connects a baby to the placenta. Blood rich in oxygen and nutrients flows through it so that the baby can grow. A baby in the womb does not eat or breathe air. Food and air reach it through the cord instead, so the cord does the jobs of a mouth and lungs.
The cord has three blood vessels: one vein and two arteries. The cord is twisted like a rope, with the two arteries winding in a spiral around the vein. The vessels are held in a soft, jelly-like cushion called Wharton's jelly. The jelly works like a built-in shock absorber, and it cushions the vessels against being squeezed, twisted or bent when the baby kicks and turns. The cord also carries waste away from the baby. The two arteries carry oxygen-poor blood back to the placenta, which takes the waste out of it, and the cleaned blood returns to the baby along the vein. The baby does not breathe air in the womb, so the placenta does the work of the lungs, and oxygen comes in through the cord.
Blood in the womb takes shortcuts. 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, and a small opening in the heart, the foramen ovale, lets blood skip 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. The baby now breathes air with its lungs, so the cord is not needed for oxygen anymore. The foramen ovale closes, usually sealing within about 3 months and leaving a dent called the fossa ovalis.
How soon should the cord be cut? Before the 1960s it was not unusual to wait five minutes or more. Then, for reasons that are unclear, doctors began clamping and cutting almost right away. The value of that quick cut was never shown. Doctors now study timing again, so the question of when to cut the cord is still being looked at carefully. As of November 2026, in the United States, the group of obstetricians called ACOG advises waiting at least 30 to 60 seconds for healthy babies, in guidance from 2016 and later. 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.
For babies born too early, waiting before clamping the cord has been linked to a lower risk of needing blood transfusions, of anemia, and of bleeding in the brain. Waiting has one small downside: it 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 to clamp when the baby needs help right away, for example if the baby has trouble breathing and needs emergency care. That baby may need treatment at once, so the pause is skipped.
After birth the cord is cut, leaving a short piece, the stump, on the baby's belly. At first the stump looks yellowish-green, and as it dries it turns brown or black, a bit like a scab. Dry, dark and crusty is normal for a stump that is healing. The stump heals much like a scab: it dries, shrinks and falls off, usually within one to three weeks after birth, though some take a bit longer. Then the belly button shows underneath, the dent we call a navel, and it stays on the body for the rest of life as a small scar.
Inside the body, the umbilical vein shrinks into a tough fibrous string called the round ligament of the liver, and the two umbilical arteries shrink to fibrous strings as well. Their far ends become the medial umbilical ligaments, which stay in the belly for life, and the near ends stay as working arteries. The belly button holds the sealed-off ends of the one vein and two arteries, and they remain as fibrous strings for the rest of life, and these leftovers are all that remains of the cord inside the body, while the belly button is the scar on the outside where the cord was attached.
Many babies are born with an umbilical hernia, a bulge at the belly button. It 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. One larger than about 2 centimeters is more likely to need surgery. Adults with an umbilical hernia usually need surgery, because it is unlikely to get better by itself.
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. 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. Nobody gets to choose an innie or an outie, because it is mostly a matter of chance, and either kind is a normal belly button. Doctors say that taping a coin over a belly button has no medical benefit and can cause a skin rash.
Mammals that grow inside their mothers are born with umbilical cords, so they have navels. On a dog or a cat the navel is a small flat scar that is hidden by fur. Marsupial mothers, such as kangaroos, do have a placenta that feeds their young before birth. Their young are born much sooner than the young of placental mammals, and they spend much less time feeding from it. Platypuses are different, because they lay eggs, so they have no umbilical cord and no belly button.
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. Early ultrasound grew out of metal flaw detectors. The Glasgow team's prototype scanner was made from a disused flaw detector, bits of Meccano toy parts and a hospital bed table. Ultrasound is now a routine part of care before birth.
Another name for the belly button is the navel, and doctors call it the umbilicus. 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. Whatever its shape, 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.
Words to know
- amnion: baby's water bag
- ligament: tough string
- hernia: bulge through muscle
- jaundice: yellow skin tint
- ultrasound: sound-wave scan
