The umbilical cord is your baby’s lifeline during pregnancy, delivering oxygen and nutrients from the placenta while carrying waste back out. It forms early, works constantly for about 33 weeks, and contains a surprisingly elegant system of blood vessels wrapped in a protective jelly-like coating. Here’s how it all works.
How the Cord Is Built
The umbilical cord contains three blood vessels: two arteries and one vein. The single vein carries oxygen-rich blood and nutrients from the placenta to the baby. The two arteries carry deoxygenated blood and waste products back from the baby to the placenta, where the mother’s body filters and replenishes it.
These three vessels are encased in a thick, slippery substance called Wharton’s jelly, a cushioning material that protects the blood vessels from compression, kinking, and damage as the baby moves around. Think of it as natural bubble wrap for a critical supply line. At full term, the cord is typically 50 to 60 centimeters long (roughly two feet) and about 12 millimeters in diameter.
Delivering Oxygen and Nutrients
Because a fetus can’t breathe, eat, or drink, the umbilical cord handles all of those jobs. Oxygen from the mother’s blood crosses the placenta and travels through the cord’s single vein into the baby’s bloodstream. Along with oxygen, the vein delivers glucose, amino acids, fatty acids, vitamins, and minerals that fuel the baby’s rapid growth.
This system kicks in remarkably early. The fetal heart starts pumping around week four of pregnancy, and by week seven the umbilical cord is fully formed and functional. From that point until birth, the cord operates as the baby’s sole connection to everything it needs to survive.
Removing Waste
The cord works in both directions. While the vein sends fresh blood to the baby, the two umbilical arteries return used blood to the placenta. This deoxygenated blood, with an oxygen saturation of roughly 40%, carries carbon dioxide and metabolic waste products the baby’s developing organs can’t yet process on their own. Once this blood reaches the placenta, the mother’s circulatory system takes over, filtering out the waste and reloading the blood with oxygen for the return trip.
Passing Along Immunity
The umbilical cord also serves as a delivery channel for the mother’s antibodies. During pregnancy, a class of protective proteins called IgG crosses the placenta and travels through the cord into the baby’s blood. These antibodies give newborns a temporary immune defense against infections the mother has already encountered or been vaccinated against, covering the gap before the baby’s own immune system is mature enough to respond on its own.
This transfer is selective. The placenta actually concentrates certain antibodies, so levels of IgG in cord blood can end up higher than in the mother’s own blood. The timing of exposure matters too. Antibodies from vaccinations given in the second trimester tend to reach higher levels in cord blood than those given very early in pregnancy, since there’s more time for the transfer to build up.
Stem Cells in Cord Blood
Beyond its day-to-day transport role, cord blood is a rich source of stem cells. These are cells capable of developing into many different types of blood and immune cells. Compared to stem cells harvested from adult bone marrow, cord blood stem cells multiply faster, have longer telomeres (a marker of cellular youth), and are more tolerant of genetic mismatches between donor and recipient. That last trait makes them especially useful in transplant medicine, where finding an exact tissue match can be difficult.
Cord blood also contains cells that help repair blood vessels, cells that reduce inflammation, and a type of immune cell that dampens the body’s tendency to reject transplanted tissue. Some families choose to bank their baby’s cord blood at birth for potential future medical use, though whether private banking is worthwhile remains a personal decision.
What Happens to the Cord After Birth
Once the baby is born, the cord still has a few final contributions to make. Current guidelines from the American College of Obstetricians and Gynecologists recommend waiting at least 30 to 60 seconds before clamping the cord. This brief delay allows extra blood to flow from the placenta into the newborn, boosting hemoglobin levels and building up iron stores that last through the first several months of life.
For premature babies, the benefits of delayed clamping are even more pronounced: better circulation during the transition to breathing air, a healthier red blood cell volume, and a lower chance of needing a blood transfusion. The final pulse of placental blood also delivers additional antibodies and stem cells that support tissue repair.
After the cord is clamped and cut, a short stump remains attached to the baby’s belly. This stump dries out, darkens, and falls off on its own within 5 to 15 days, leaving behind what becomes the navel.
When the Cord Causes Problems
Most umbilical cords function without any issues, but a few complications can arise. A nuchal cord, where the cord wraps around the baby’s neck, is common and usually harmless since the cord’s protective jelly prevents the vessels from being squeezed shut. True knots in the cord are rarer and can restrict blood flow if they tighten during delivery.
The most serious complication is cord prolapse, where the cord slips through the cervix ahead of the baby. This is a rare emergency. When the baby’s body presses against the cord during delivery, it can cut off blood flow and oxygen. The estimated fetal mortality rate for cord prolapse is under 10% in hospital settings, but that risk increases 18-fold when it happens outside a hospital. Premature and low-birth-weight babies face roughly double the mortality risk. Cord prolapse is typically detected by sudden changes in the baby’s heart rate during labor and requires immediate delivery.