Look at your belly button. It is a scar, and it is left over from the umbilical cord. It marks the spot where your cord was attached to you before you were born. The cord was a lifeline that connected a baby to the placenta before birth. So the mark sits there to this day.

Before birth, food and air reach a baby through the cord. It does not eat or breathe air in the womb. Inside the cord are three blood vessels. One brings in blood with oxygen and food. The other two carry used blood and waste back to the placenta. The cord is twisted like a rope, as the two wind in a spiral around the first.

After birth, a newborn breathes air with its lungs, so the cord is not needed for air anymore. The cord is clamped, which means pinched shut, and then cut. What is left is a short stump. The stump dries up and falls off, within one to three weeks after birth. Then the stump is gone for good.

While the stump is healing, it should be kept clean and dry. Adults are told not to pull or tug at it, even if it looks like it is barely hanging on. It should be left alone to fall off by itself. They do not need to put alcohol, lotion or anything else on it. If it gets dirty, they rinse it with plain water.

Red or swollen skin around the base of the cord can be a sign of infection. If a kid ever sees a stump that is red, swollen, smelly or bleeding, the kid should tell an adult, and a doctor should check it. A stump that has not fallen off by about 3 weeks should be seen by a doctor too.

When the stump falls off, it leaves behind the dent we call a navel. Inside, the opening of the cord heals and seals itself shut. After that, the cord has done its job. All that is left to see is the mark, right where the cord used to be. The cord is gone, but the mark stays.

An innie is a little dent, and an outie sticks out. An innie sinks in, with the skin around it folding inward. An outie often has a little extra scar tissue. Most are innies. Outies are less common. Nobody gets to choose which one they get. It is mostly a matter of chance, so no one picks.

Many babies are born with a hernia, a bulge at the belly button. It heals by itself, most often in the first 12 to 18 months of life. Most hernias in children go away on their own, by about age 5. So it is not rare, and it often goes away with no help at all, long before the child is grown.

Most mammals have belly buttons, but you may not see them. On a dog or a cat the navel is a small flat scar that is hidden by fur. Chimpanzees and gorillas have ones that look a lot like ours. Monotremes, such as the platypus and echidna, lay eggs, and their young hatch from eggs. So platypuses have no cord and no belly button.

Words to know

• umbilical: of the cord
• stump: bit of cord
• navel: belly button
• hernia: a bulge
• monotremes: egg-laying mammals

The short version

A belly button is a scar left where the umbilical cord was attached before birth. The cord carried oxygen and food to the baby. After birth the cord is cut, and the stump dries and falls off in one to three weeks, leaving the navel.

Quick facts

• The cord is clamped, which means pinched shut, and then cut.
• The stump should be kept clean and dry and left to fall off by itself.
• Most belly buttons are innies, and nobody gets to choose.
• Most umbilical hernias in children go away by about age 5.
• Platypuses lay eggs, so they have no umbilical cord and no belly button.