Your belly button is your very first scar. It is a scar left over from the umbilical cord, and it marks the spot where that cord was attached. Your navel is what remains of the connection between you and your mother before you were born. In other words, the place where you were once joined to her is the place where your belly button is now.

Before a baby is born, it does not eat or breathe air, because food and oxygen reach the baby through the umbilical cord instead. The cord is a lifeline that connects the baby to the placenta before birth. Through the cord, blood rich in oxygen and nutrients flows from the placenta to the baby so that the baby can grow, and that supply is how a baby gets what it needs in the womb.

At full term the cord is about 55 to 60 centimeters long, around 22 inches, but only 1 to 2 centimeters thick. At full term it holds three blood vessels, one vein and two arteries. The arteries wind around the vein in a spiral, so the cord is twisted like a rope. The vessels sit in a soft cushion called Wharton's jelly.

The cord also carries waste away from the baby. Blood goes back along the two arteries to the placenta, which takes the waste out of it, and the cleaned blood returns to the baby along the vein. The arteries carry oxygen-poor blood on that trip. Because the baby does not breathe air in the womb, the placenta does the work of the lungs, and oxygen comes in through the cord.

Everything changes at birth. After the baby is born, the baby breathes air with its lungs, so the cord is no longer needed for oxygen. The first breaths fill the lungs with air, and blood then starts to flow to the lungs instead of taking fetal shortcuts. The change begins within seconds, which makes birth a very fast switch for the baby's body.

After birth, the cord's blood vessels inside the body close up. 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. After the cord is cut, the opening inside it also heals and seals itself shut, so what is left in the belly is closed up for good.

After birth the cord is cut, leaving a short piece called the stump on the baby's belly. The stump heals much like a scab: it dries, shrinks and falls off, usually within one to three weeks after birth. When the stump falls off, it leaves behind the dent we call a navel, and the belly button shows underneath.

A stump heals best when it is left alone, and grown-ups are told not to pull or tug at it. They fold the top of a diaper down so the stump stays dry, and they give sponge baths instead of tub baths. They do not need to put alcohol or lotion on it, only rinse it with plain water if it gets dirty. If a baby's stump has not fallen off by about 3 weeks, the baby should see a doctor. And if you ever see a stump that is red, swollen, smelly or bleeding, tell a grown-up.

Then there is the question that everyone eventually asks, whether a belly button is an innie or an outie. An innie sinks in, with the skin around it folding inward. An outie sticks out, often because of a little extra scar tissue. Most belly buttons are innies. Nobody gets to choose, and it is mostly a matter of chance. It may depend on how the skin heals after the stump falls off. Either way, how a belly button looks varies widely from person to person.

People are not the only ones. Dogs, elephants, lions, bears and many other mammals grow inside their mothers and get food through a placenta and a cord, so they have navels too. On furry animals the small flat scar is usually hidden by hair. The platypus lays eggs, so it has no cord and no belly button at all.

A study of 60 volunteers swabbed their belly buttons and found 2,368 kinds of bacteria. On average each belly button had about 67 kinds, though some had as few as 29 and some as many as 107. Scientists compared belly buttons to rainforests: only a handful of bacteria types were found on most people.

Words to know

• umbilical: of the cord
• placenta: acts as lungs
• stump: leftover cord piece
• navel: belly button
• platypus: egg-laying mammal