The umbilical cord is the physical lifeline between a pregnant person and the developing fetus. It delivers oxygen and nutrients from the placenta to the baby and carries waste products like carbon dioxide back out. From the earliest weeks of pregnancy through the moment of birth, the cord handles every function that a baby’s lungs, digestive system, and kidneys will eventually take over on their own.
How the Cord Works
The umbilical cord is a flexible, tube-like structure that connects the fetus to the placenta, the organ attached to the uterine wall. The placenta pulls oxygen and nutrients from the parent’s blood supply, and the cord shuttles those resources to the baby. At the same time, the baby’s blood carries carbon dioxide and metabolic waste back through the cord to the placenta, where it passes into the parent’s circulation and gets processed out normally. The baby never breathes, eats, or filters waste on its own until after birth. The cord does all of it.
Blood flows in two directions simultaneously. Oxygen-rich, nutrient-loaded blood travels from the placenta to the fetus through a single large vein. Oxygen-poor blood carrying waste flows from the fetus back to the placenta through two smaller arteries. So despite being a single tube, the cord contains three distinct blood vessels working at once.
What the Cord Looks Like
An average umbilical cord is about 55 centimeters long (roughly 22 inches) and 1 to 2 centimeters in diameter. It has a natural spiral shape, typically coiling about 11 times along its length. The three blood vessels inside are cushioned by a thick, slippery connective tissue called Wharton’s jelly, which protects the vessels from compression or kinking as the baby moves around in the womb. That jelly, along with the cord’s coiled structure, gives it a surprising amount of durability. A baby can roll, kick, and somersault without cutting off its own blood supply.
What the Cord Delivers
Everything the fetus needs to grow passes through the umbilical cord. That includes oxygen (since the baby’s lungs are filled with fluid, not air), glucose for energy, amino acids for building tissue, fats, vitamins, minerals, and antibodies from the parent’s immune system. The cord also transfers hormones that support fetal development.
On the return trip, the two arteries carry carbon dioxide and other waste products the fetus generates through normal metabolism. Once that blood reaches the placenta, the waste crosses into the parent’s bloodstream. The parent’s lungs exhale the carbon dioxide, and the kidneys and liver process the rest. In a very real sense, the parent’s organs do double duty throughout pregnancy.
Common Cord Complications
A nuchal cord, where the umbilical cord wraps around the baby’s neck, is one of the most common concerns parents hear about. It occurs in roughly 10% to 29% of pregnancies. Up to 1 in 4 babies is born with a single loop around the neck, and about 2% to 8% have two or more loops. Despite how alarming it sounds, a loose nuchal cord very rarely causes problems. Most babies born with one are perfectly healthy.
A tight nuchal cord is more likely to cause issues, though still uncommon. It can temporarily slow the baby’s heart rate during labor, which providers watch for on fetal heart rate monitors. In some cases, a tight cord leads to minor complications after birth: small broken blood vessels in the baby’s eye, pinpoint-sized spots of bleeding on the face, or brief breathing difficulties. These typically resolve quickly. In rare situations where a tight nuchal cord restricts blood flow enough to cause fetal distress, the delivery team may reposition the baby during birth or perform an emergency cesarean section.
What Happens to the Cord After Birth
Once a baby is born and starts breathing on its own, the umbilical cord is no longer needed. Current guidelines from both the American College of Obstetricians and Gynecologists and the American Academy of Pediatrics recommend waiting at least 30 to 60 seconds after birth before clamping the cord in healthy, full-term infants. This delay allows extra blood to transfer from the placenta to the baby, boosting the newborn’s iron stores and blood volume.
After the cord is clamped and cut, a short stump remains attached to the baby’s belly button. That stump dries out and falls off on its own, usually within 5 to 15 days. During that time, you should keep the stump clean with gauze and water, give your baby sponge baths instead of tub baths, and let the stump separate naturally rather than pulling at it. Once it falls off, what’s left is the navel.
Cord Blood and Stem Cells
The blood remaining in the umbilical cord after birth contains a rich supply of stem cells, which are cells capable of developing into many different types of blood and immune cells. These stem cells are already used in transplants to treat cancers like leukemia and lymphoma, bone marrow diseases, sickle cell anemia, and certain immune system disorders. Some families choose to bank their baby’s cord blood at birth, either in a private bank for potential family use or in a public bank where it can be matched to patients in need. Collection is painless and happens after the cord is cut, so it doesn’t affect the birth or the baby.