The umbilical cord is made of three blood vessels, a protective jelly-like tissue called Wharton’s jelly, and an outer membrane. An average cord at birth is about 55 cm long and 1 to 2 cm in diameter, with a distinctive spiral shape that helps it resist kinking and compression throughout pregnancy.
The Three Blood Vessels
Most umbilical cords contain exactly three blood vessels: one vein and two arteries. The vein is the largest of the three and carries oxygen and nutrient-rich blood from the placenta to the baby. The two smaller arteries work in the opposite direction, carrying waste products and oxygen-depleted blood back from the baby to the placenta for filtering. This setup is the reverse of how arteries and veins work in the rest of your body, where arteries typically carry oxygenated blood.
About 1% of pregnancies have a cord with only a single artery instead of two, a variation called a single umbilical artery. In most cases this doesn’t cause problems, though doctors monitor these pregnancies more closely because of a slightly higher chance of associated complications.
Wharton’s Jelly: The Cord’s Protective Filling
The bulk of the umbilical cord is made of Wharton’s jelly, a thick, slippery connective tissue that surrounds and cushions the blood vessels. First described in 1656 by the anatomist Thomas Wharton, this substance acts as a biological shock absorber. When the baby moves, rolls, or compresses the cord, the jelly distributes pressure evenly so blood flow isn’t interrupted.
Wharton’s jelly is built from a mesh of proteins and sugar molecules. The main structural protein is collagen, primarily types I and III, which together make up about 87% of the collagen in the jelly. A less common form, type V collagen, accounts for roughly 12%, which is unusually high compared to other tissues in the body. This collagen framework gives the jelly its strength and flexibility.
Woven through that collagen is hyaluronic acid, a molecule that traps large amounts of water. It makes up nearly 70% of the sugar-based molecules in Wharton’s jelly and is the main reason the tissue feels slippery and gel-like. The remaining sugar molecules, including small amounts of keratan sulfate and chondroitin sulfate, each contribute only a few percent. Together, these components create a tissue that is firm enough to hold the blood vessels in place but soft enough to absorb constant movement and pressure.
Why the Cord Spirals
If you’ve ever looked at an umbilical cord, the first thing you notice is that it twists like a telephone cord. A typical cord has about 11 of these spiral turns, or helices. In uncomplicated pregnancies, the average is roughly 0.17 coils per centimeter of cord length. When the number exceeds 3 coils per 10 cm, the cord is considered hypercoiled, which can occasionally affect blood flow.
The spiraling comes from the umbilical arteries themselves. Unlike nearly every other artery in the body, the umbilical arteries have two layers of muscle in their walls, and these layers run in opposite directions. As the muscles contract, they naturally twist the vessels into helices, pulling the surrounding Wharton’s jelly and the outer membrane along with them. This coiling serves a mechanical purpose: a spiraled cord is much harder to kink or flatten than a straight one, similar to how a coiled garden hose resists bending more than a straight tube.
The Outer Membrane
Wrapping around everything is a thin, smooth layer called the amnion. This is the same membrane that lines the inside of the amniotic sac surrounding the baby. It forms a tough, waterproof sleeve that keeps the cord’s internal structure intact and protected within the amniotic fluid. Though only a single cell layer thick in places, it prevents the jelly from dissolving into the surrounding fluid and keeps the cord as a self-contained unit.
How Blood Flows Through the Cord
Blood moves through the umbilical cord at a surprisingly high rate. During the second and early third trimester, roughly 120 milliliters of blood flow through the umbilical vein every minute for each kilogram of the baby’s estimated weight. That rate gradually decreases toward the end of pregnancy, dropping to about 90 mL per minute per kilogram by 40 weeks. For a 3.5 kg baby at full term, that translates to over 300 mL of blood cycling through the cord every minute.
This flow is maintained by the baby’s heart pumping blood through the two arteries and the placenta pushing oxygenated blood back through the single, larger vein. The Wharton’s jelly, the spiral structure, and the amnion all work together to keep these vessels open and unobstructed, even when the baby somersaults or the cord gets temporarily compressed during contractions.
Stem Cells in the Cord
Both the cord blood and the cord tissue itself contain stem cells, which is why umbilical cord banking has become increasingly common. The blood inside the cord’s vein is rich in blood-forming stem cells, the same type found in bone marrow, which can develop into red blood cells, white blood cells, and platelets. These are already used in treatments for certain blood cancers and immune disorders.
Wharton’s jelly contains a different type: mesenchymal stem cells, which have the potential to develop into bone, cartilage, fat, and other connective tissues. These cells are of particular interest because the jelly provides a large supply of them, and they can be collected without any invasive procedure, since the cord is discarded after birth. The combination of both cell types in a single organ is part of what makes the umbilical cord valuable beyond its nine months of use.