A belly button is a scar that is left over from the umbilical cord, and it marks the spot where the cord was attached before the baby was born. The cord is a lifeline that connects a baby to the placenta, and it has three blood vessels, one vein and two arteries. Through the cord, blood rich in oxygen and nutrients flows from the placenta to the baby so that the baby can grow. At full term the cord is about 55 to 60 centimeters long and 1 to 2 centimeters thick, and it is twisted like a rope, with the two arteries winding in a spiral around the vein.

The cord forms early in pregnancy, within the first weeks, from small stalks that connect the embryo to its supply, including the yolk stalk and the allantois. Later the growing amniotic sac wraps around these stalks, which is how the cord gets its covering of amnion. Inside that thin tube-like sheath, the blood vessels are held in a soft, jelly-like cushion called Wharton's jelly, which works like a built-in shock absorber and cushions the vessels against being squeezed, twisted or bent when the baby kicks and turns.

Before birth the blood travels in two directions along the cord, and it takes several shortcuts through the baby's body. 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. A baby in the womb does not eat or breathe air, so the placenta does the work of the lungs. The ductus venosus lets a large share of the blood from the cord skip the liver and flow straight toward the heart, while the foramen ovale, a small opening in the heart, and the ductus arteriosus, a short blood vessel, both let 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 the fetal shortcuts. The change begins within seconds. The foramen ovale usually seals within about 3 months and leaves a dent called the fossa ovalis, and breathing makes the muscle of the ductus arteriosus tighten for good within 10 to 15 hours, so that it becomes a fibrous string, the ligamentum arteriosum. The ductus venosus closes and turns into the ligamentum venosum. 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. The belly button holds the sealed-off ends of the one vein and two arteries, and they remain as strings for the rest of life.

After birth the cord is cut, leaving a short piece, the stump, on the baby's belly. The stump heals much like a scab, since it dries, shrinks and falls off, usually within one to three weeks after birth, and the dent we call a navel shows underneath. While it is healing, the stump should be kept clean and dry, and grown-ups are told not to pull or tug at it, even if it looks like it is barely hanging on. The opening inside the cord also heals and seals itself shut. 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 that.

Before the 1960s it was not unusual to wait five minutes or more to cut the cord. Then, for reasons that are unclear, doctors began clamping and cutting almost right away, and the value of that quick cut was never shown, so doctors now study timing again. 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. Waiting lets extra blood move from the placenta to a full-term baby, which gives the baby more iron stored in the body in the first months of life. It may slightly raise the chance of newborn jaundice, which hospitals that delay clamping are set up to monitor, and doctors do not wait when a baby needs help right away.

Doctors have studied the cord by slicing it very thin and looking at it under a microscope, which shows the cord's two arteries, one vein and the Wharton's jelly that fills the space around them. They have also learned to look at babies before birth. 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 the paper by Ian Donald, John McVicar and Tom Brown held the first ultrasound pictures of a baby before birth. Early ultrasound grew out of metal flaw detectors, and the Glasgow team's prototype scanner was made from a disused flaw detector, bits of Meccano toy parts and a hospital bed table.

The navel looks like a pushed-in area with a rim around it, and scientists have also studied it as a home for bacteria. A study of 60 volunteers found 2,368 kinds of bacteria in their belly buttons. On average, each belly button had about 67 kinds, 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. Scientists compared belly buttons to rainforests because only a handful of bacteria types were found on most people.

An innie is a little dent in the belly, and an outie sticks out. Most belly buttons are innies, and outies are less common. No one knows exactly why some belly buttons are outies, although 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. Nobody gets to choose, because the way a belly button looks is mostly chance. Whatever its shape, a belly button is a scar left over from the umbilical cord.

Words to know

• ligamentum: tough string
• allantois: early cord stalk
• arteriosus: artery-like
• fibrous: stringy
• jaundice: yellow tint