The umbilical cord is your baby’s lifeline during pregnancy, acting as a two-way supply route between the placenta and the fetus. It contains three blood vessels: one vein that delivers oxygen and nutrients from the placenta to the baby, and two arteries that carry carbon dioxide and metabolic waste back to the placenta for the mother’s body to dispose of. This exchange begins remarkably early, around the fourth week of pregnancy, when the fetal heart starts pumping.
What’s Inside the Cord
A mature umbilical cord at birth is roughly 50 to 60 centimeters long (about two feet) and 12 millimeters thick. Inside, the single vein and two arteries are embedded in a thick, slippery substance made of collagen fibers, hyaluronic acid, and other compounds that give it a gel-like consistency. This protective padding surrounds the blood vessels and shields them from being pinched or compressed as the baby moves, rolls, and kicks throughout pregnancy. Think of it like the foam insulation around pipes: it keeps the vessels open and blood flowing even when the cord bends or gets squeezed.
The cord takes its final shape near the end of the first trimester, after earlier embryonic structures have regressed. By that point, the two arteries and one vein are fully established and will continue functioning until delivery.
How Blood Flows in Two Directions
The blood flow inside the cord runs counterintuitively compared to how we normally think about veins and arteries. In the rest of your body, arteries carry oxygen-rich blood and veins carry oxygen-poor blood. In the umbilical cord, it’s reversed. The single vein carries freshly oxygenated, nutrient-loaded blood from the placenta to the fetus. The two arteries carry the baby’s deoxygenated blood, loaded with carbon dioxide and waste products, back to the placenta.
This isn’t a slow trickle. During the third trimester, blood moves through the umbilical arteries at roughly 143 milliliters per kilogram of fetal weight per minute. For a baby weighing around 3 kilograms near term, that translates to over 400 milliliters of blood cycling through the cord every minute.
Oxygen and Nutrient Delivery
Because a fetus can’t breathe or eat, the umbilical cord handles both jobs. Oxygen from the mother’s blood crosses the placenta into the fetal blood supply and travels through the umbilical vein to the baby. Glucose and other nutrients follow the same path. The placenta acts as the transfer point, not the cord itself. The cord is purely the highway; the placenta is where the exchange between maternal and fetal blood actually happens. The two blood supplies never directly mix.
On the return trip, the two umbilical arteries transport carbon dioxide and a range of metabolic byproducts back to the placenta. Research analyzing the chemical makeup of arterial cord blood has found elevated levels of compounds like hypoxanthine and hydroxybutyric acid, both normal byproducts of fetal metabolism. Once these waste products reach the placenta, they pass into the mother’s bloodstream and are filtered out through her lungs, kidneys, and liver.
What Happens at Birth
The cord doesn’t stop working the moment a baby is born. Blood continues flowing from the placenta to the newborn for several minutes after delivery. Physiological studies show that about 80 milliliters of blood transfers from the placenta by one minute after birth, reaching roughly 100 milliliters by three minutes. The baby’s first breaths actually help pull this blood across.
This is why most medical organizations now recommend waiting before clamping the cord. The American College of Obstetricians and Gynecologists recommends delaying clamping for at least 30 to 60 seconds in healthy term and preterm babies. The World Health Organization recommends waiting at least one minute. Some organizations, including the American College of Nurse-Midwives, recommend waiting two to five minutes.
The benefits of this extra blood transfer are well documented. In full-term babies, delayed clamping raises hemoglobin levels at birth and builds up iron stores during the first several months of life, which can support healthy development. In preterm babies, the benefits are even more pronounced: better circulation during the transition to breathing air, a healthier red blood cell volume, fewer blood transfusions, and lower rates of serious complications like intestinal injury and brain bleeding.
When the Cord Has Two Vessels Instead of Three
The most common cord variation is a single umbilical artery, where the cord has only one artery instead of two. This occurs in roughly 0.5 to 6 percent of pregnancies, depending on the population studied. In most cases, the remaining artery compensates and the baby develops normally. When a single umbilical artery is detected on ultrasound, providers typically monitor the pregnancy more closely and may recommend additional imaging to check for other differences, but the finding alone is not cause for alarm.
Cord length also varies. Anything between about 36 and 84 centimeters is considered normal. Cords shorter than 30 centimeters or longer than 100 centimeters are more likely to be associated with delivery complications, including a higher chance of cesarean delivery.
Stem Cells in the Cord
Beyond its role during pregnancy, the umbilical cord is a rich source of stem cells, which are cells capable of developing into many different tissue types. These cells are found in both the cord blood and the gel-like tissue surrounding the vessels. They’ve been used clinically for over a decade to treat bone marrow disorders and inherited metabolic conditions. In lab and animal research, cord-derived stem cells have shown the ability to develop into nerve cells, heart muscle, bone, and cartilage, with some early studies exploring their potential in conditions like Parkinson’s disease and stroke recovery.
Cord blood banking, whether through public donation or private storage, captures these cells at birth for potential future use. The cells are considered particularly valuable because they’re young, adaptable, and less likely to trigger immune rejection compared to stem cells from adult donors.