Yes, the umbilical cord is the sole source of oxygen for a developing baby throughout pregnancy. It works much like a breathing tube, delivering oxygen-rich blood from the placenta to the fetus and carrying oxygen-depleted blood back. Since a fetus can’t breathe air in the womb, this system replaces the function of the lungs entirely until birth.
How Oxygen Reaches the Fetus
The process starts in the placenta, where maternal and fetal blood come extremely close together but never actually mix. Tiny specialized structures called vasculosyncytial membranes thin the tissue barrier between the two blood supplies down to just 2 to 3 micrometers, roughly the width of a single red blood cell. Oxygen moves across this thin barrier from the mother’s blood into the baby’s blood purely through diffusion, driven by the difference in oxygen concentration on each side.
Several biological tricks make this transfer remarkably efficient. Fetal blood contains a special form of hemoglobin (the protein in red blood cells that carries oxygen) with a significantly higher affinity for oxygen than the adult version. This means fetal blood grabs onto oxygen more readily than the mother’s blood releases it, essentially pulling oxygen across the barrier. At the same time, a process called the Bohr-Haldane effect works in the baby’s favor: as the mother’s blood absorbs carbon dioxide waste from the fetus, it becomes slightly more acidic, which causes it to release even more oxygen. The two exchanges reinforce each other.
Inside the Umbilical Cord
A fully formed umbilical cord, which reaches its final structure around the 12th week of pregnancy, contains three blood vessels: one vein and two arteries. Their roles are the reverse of what you might expect. The single umbilical vein carries oxygen-rich blood from the placenta toward the baby. The two umbilical arteries carry oxygen-depleted blood and waste products back to the placenta. This is the opposite of how veins and arteries work in the rest of the body, where arteries carry oxygenated blood and veins carry deoxygenated blood.
All three vessels are cushioned in a thick, gel-like substance called Wharton’s jelly, which prevents the cord from being compressed, twisted, or bent in ways that would cut off blood flow. The entire structure is wrapped in a layer of membrane from the amniotic sac. This design is critical because any sustained interruption in flow means the baby loses its oxygen supply.
How Much Oxygen the Cord Delivers
Blood flow through the umbilical vein increases dramatically as the baby grows. At 20 weeks of pregnancy, about 63 milliliters of blood passes through the vein each minute. By 38 weeks, that volume rises to more than 370 milliliters per minute, roughly a can of soda’s worth of blood flowing every 60 seconds. The pressure inside the umbilical vein also rises from about 4.5 mmHg at 18 weeks to 6 mmHg by the end of pregnancy, helping push blood toward the baby.
Despite this robust flow, fetal blood oxygen levels are much lower than what you’d see in a breathing person. The average oxygen saturation in the umbilical vein (the oxygen-rich side) is about 63%, and in the umbilical arteries (returning blood) it’s around 24%. For comparison, a healthy adult’s blood oxygen saturation typically sits between 95% and 100%. The baby’s body is built to function in this lower-oxygen environment, which is one reason fetal hemoglobin is so much better at binding oxygen than the adult version.
How the Baby Prioritizes Oxygen
Once oxygen-rich blood enters the baby through the umbilical vein, it doesn’t simply circulate the way adult blood does. A small vessel called the ductus venosus acts as a shortcut, channeling a portion of that oxygen-rich blood directly past the liver and into the heart. The trumpet-like shape of this vessel accelerates the blood, giving it enough momentum to stream upward through the right side of the heart and across a small opening called the foramen ovale into the left side.
This routing is deliberate. The left side of the heart pumps blood to the coronary arteries and up to the brain first, ensuring the two most oxygen-hungry organs get the richest blood supply. It’s an elegant system of prioritization that disappears after birth, when the baby begins breathing air and the ductus venosus and foramen ovale gradually close on their own.
What Happens When Flow Is Disrupted
Because the umbilical cord is the baby’s only oxygen source, anything that restricts blood flow can cause fetal hypoxia, a dangerous drop in oxygen levels. This can result from mechanical compression of the cord (such as the cord being wrapped tightly around the baby’s neck or becoming trapped during delivery), a blood clot within the cord vessels, or in rare cases, cord prolapse, where the cord slips ahead of the baby during labor and gets compressed against the birth canal.
Babies born with the cord around their neck (nuchal cord) or born post-term tend to show lower umbilical cord oxygen saturation values at delivery, indicating a reduced margin of safety during the birth process. Monitoring fetal heart rate patterns during labor is one of the primary ways clinicians detect signs of cord compression, since changes in heart rate are often the first visible indicator that oxygen delivery has been interrupted.
The Transition at Birth
The umbilical cord continues to deliver oxygen for a short time after the baby is born, even as the newborn takes its first breaths. This overlap period is one reason the American College of Obstetricians and Gynecologists recommends waiting at least 30 to 60 seconds before clamping the cord in healthy term and preterm infants. During this window, the baby receives additional blood from the placenta, which helps establish a stronger red blood cell volume and supports the transition from placental to lung-based breathing.
For preterm infants, delayed clamping offers particular benefits, including improved circulation during the critical first minutes of life and a reduced need for blood transfusions afterward. Once the cord is clamped and cut, the baby’s lungs take over gas exchange permanently, and the umbilical vessels that served as a lifeline for nine months begin to close and eventually become ligaments in the abdomen.