The umbilical cord is your baby’s lifeline during pregnancy, delivering oxygen and nutrients from the placenta while carrying waste products back out. It forms by around 12 weeks of gestation, typically reaches 50 to 60 cm in length at full term, and contains three blood vessels embedded in a protective jelly-like tissue.
How the Cord Is Built
The umbilical cord contains two arteries and one vein, all surrounded by a soft, moist connective tissue called Wharton’s jelly. The entire structure is wrapped in a single layer of the same membrane that lines the amniotic sac. Wharton’s jelly acts as a natural cushion, protecting the blood vessels from compression or kinking as the baby moves around. Toward the end of pregnancy, the cord’s diameter gradually decreases as the water content in this jelly declines.
Delivering Oxygen and Nutrients
Since a fetus can’t breathe, eat, or drink, the umbilical cord handles all three jobs at once. The single umbilical vein carries freshly oxygenated, nutrient-rich blood from the placenta into the baby’s circulation. This blood contains everything the baby needs to grow: glucose, amino acids, fatty acids, vitamins, and minerals, all filtered from the mother’s bloodstream through the placenta’s thin membrane.
The two umbilical arteries work in the opposite direction, carrying deoxygenated, nutrient-depleted blood from the baby back to the placenta. There, carbon dioxide and other metabolic waste diffuse out of the baby’s blood and into the mother’s circulation, where her lungs and kidneys handle disposal. The cord essentially functions as the baby’s lungs, digestive system, and kidneys all rolled into one flexible tube.
Transferring Immune Protection
The cord doesn’t just carry food and oxygen. It also delivers maternal antibodies that give the baby a head start on fighting infections. IgG, the most abundant antibody type in the blood, is the only antibody class that crosses the placenta in significant amounts. This transfer begins as early as 13 weeks of gestation but ramps up dramatically in the final stretch.
Between weeks 17 and 22, fetal antibody levels sit at only 5% to 10% of the mother’s levels. By weeks 28 to 32, they reach about 50%. A sharp increase occurs after week 36, and by full term, the baby’s IgG concentration actually exceeds the mother’s by 20% to 30%. This passive immunity helps protect newborns during their first months of life, while their own immune systems are still learning to produce antibodies.
Hormones and Signaling
Hormones also travel through the umbilical cord in both directions. The placenta produces hormones like corticotropin-releasing hormone (CRH) that influence fetal development, and the baby’s own adrenal hormones, including cortisol, circulate back through the cord. These chemical signals help regulate growth, brain development, and the maturation of fetal organs in preparation for life outside the womb. Certain medications the mother takes can also cross the placenta and reach the baby through this same pathway.
What Happens at Birth
Once the baby is born and begins breathing, the umbilical cord’s job is essentially done. But the timing of when it’s clamped matters. The World Health Organization recommends delaying cord clamping until at least one minute after birth, or until the cord stops pulsing. During this short window, blood continues flowing from the placenta to the baby, providing an extra boost of iron-rich blood that can improve the infant’s iron stores for up to six months.
Stem Cells in Cord Blood
The blood remaining in the umbilical cord after birth contains valuable stem cells. These include blood-forming stem cells similar to those found in bone marrow, as well as a different type of stem cell found in the cord tissue itself, particularly within the Wharton’s jelly surrounding the vessels. Cord blood has been used therapeutically for patients with bone marrow disorders and inherited metabolic conditions.
Researchers have also found that stem cells from the cord tissue can be coaxed into becoming bone, cartilage, heart muscle, and nerve cells in laboratory settings. These cells appear to gravitate toward damaged or rapidly growing tissues and may support healing by releasing growth-promoting factors. One practical challenge with cord blood transplants has been collecting enough cells to treat an adult patient, but combining different stem cell types from the cord may help overcome this limitation.
Common Cord Variations
The cord occasionally develops differently than expected. A nuchal cord, where the umbilical cord wraps around the baby’s neck, occurs in roughly 20% to 35% of full-term singleton deliveries. That number sounds alarming, but single and even double nuchal cords are overwhelmingly delivered without any harm to the baby. Most newborns with a nuchal cord show no signs of oxygen deprivation or poor outcomes. The risk rises only in rare cases of three or more loops or when a true knot forms in the cord, which is associated with a 4- to 10-fold increased risk of stillbirth.
About 0.6% to 1% of newborns have a single umbilical artery instead of the usual two. This is technically the most common congenital anomaly in humans. In roughly 85% of these cases, the single artery is an isolated finding with no associated problems. In the remaining 15%, it occurs alongside other structural or chromosomal differences, which is why it’s typically flagged during prenatal ultrasounds for further evaluation.