The umbilical cord is attached to the baby’s belly on one end and the placenta on the other. It serves as the lifeline between a developing baby and the organ that supplies it with oxygen, nutrients, and waste removal throughout pregnancy. The spot where the cord meets the baby’s abdomen becomes the belly button after birth.
The Two Attachment Points
On the baby’s side, the cord connects directly to the abdomen at a site that will eventually become the navel. Blood vessels from the cord pass through the abdominal wall and link into the baby’s circulatory system, carrying oxygen-rich blood in and waste products out.
On the other end, the cord inserts into the placenta, a temporary organ that grows inside the uterus during pregnancy. The placenta is attached to the uterine wall and acts as an exchange station: the mother’s blood delivers oxygen and nutrients to the placenta, and the cord carries those supplies to the baby. It also works in reverse, transporting carbon dioxide and waste from the baby back through the cord to the placenta, where the mother’s body filters them out. The two blood supplies never mix directly.
What’s Inside the Cord
The umbilical cord contains three blood vessels: one vein and two arteries. The vein carries oxygen and nutrient-rich blood from the placenta to the baby, while the two arteries carry deoxygenated blood and waste back to the placenta. This is the reverse of how arteries and veins work everywhere else in the body, which sometimes surprises people.
These vessels are surrounded by a thick, cushioning tissue that protects them from compression. This protective layer keeps blood flowing freely even as the baby moves, kicks, and shifts position in the womb. It works by absorbing pressure and distributing force away from the blood vessels, essentially acting as a built-in shock absorber. Without it, the baby’s own movements could pinch off its blood supply.
An average umbilical cord is about 55 cm (roughly 22 inches) long and 1 to 2 cm in diameter, with a natural spiral or coil along its length. About 5% of cords are shorter than 35 cm, and another 5% stretch longer than 80 cm.
Where the Cord Meets the Placenta
The cord typically inserts near the center of the placenta, but the exact landing spot varies. Doctors describe the insertion as central, off-center (eccentric), marginal, or velamentous depending on how close it is to the edge. Central and off-center insertions account for over 90% of pregnancies, and off-center is actually the most common position.
Marginal insertion means the cord attaches at or very near the edge of the placenta. Velamentous insertion, the rarest type, means the cord doesn’t reach the placenta at all. Instead, it attaches to the surrounding membranes, forcing the blood vessels to travel unprotected across the membrane surface before reaching the placenta. Without that cushioning tissue around them, these exposed vessels are more vulnerable to tearing and bleeding.
Velamentous insertion occurs in about 1% of single-baby pregnancies and roughly 6% of twin pregnancies. For twins sharing a single placenta, the rate rises to about 15%. When it happens, there’s an increased risk of slower fetal growth, premature birth, and placental separation from the uterine wall. If exposed blood vessels end up near the cervix, a planned cesarean delivery dramatically improves outcomes, with fetal survival rates reaching 97% to 99%.
When the Cord Has Two Vessels Instead of Three
A small number of babies develop a cord with only one artery instead of the usual two. This is called a single umbilical artery, and it’s typically detected during a routine ultrasound. When it occurs in isolation, with no other abnormalities, the baby usually develops normally. However, about 10% of these babies experience slower growth compared to 4.4% with a standard three-vessel cord.
When a single umbilical artery is found alongside other abnormalities on ultrasound, the picture changes. About half of fetuses diagnosed with a single umbilical artery before birth have associated chromosomal or structural differences, most commonly affecting the heart, kidneys, or nervous system. For isolated cases, kidney abnormalities show up in about 7% to 9% of infants, though only about 4% of those need treatment.
What Happens to the Cord After Birth
Once the baby is born and begins breathing on its own, the umbilical cord is no longer needed. Current guidelines from the World Health Organization recommend waiting at least one minute after birth before clamping the cord. This delay allows additional blood to flow from the placenta to the newborn, which can improve the baby’s iron stores for up to six months.
After clamping and cutting, a small stump remains on the baby’s abdomen. This stump dries out, darkens, and falls off on its own, typically within one to three weeks. What’s left behind is the belly button. Inside the body, the blood vessels that once ran through the cord gradually close and become ligaments, permanent remnants of a connection that’s no longer needed.