A normal human umbilical cord contains exactly three blood vessels: two arteries and one vein, all embedded in a soft, protective connective tissue called Wharton’s jelly.1PubMed Central. The development, structure and blood flow within the umbilical cord with particular reference to the venous system That three-vessel arrangement reaches its final form by about the twelfth week of pregnancy, but the path to getting there involves the loss of a fourth vessel, and the anatomy of what remains is unlike anything else in the human body.
Why the Blood Flow Seems Backward
One thing that trips people up is the direction of blood flow. In every other part of the body, arteries carry oxygen-rich blood away from the heart while veins return oxygen-poor blood. The umbilical cord flips that convention. The two umbilical arteries carry deoxygenated, waste-laden blood from the fetus to the placenta. The single umbilical vein carries freshly oxygenated, nutrient-rich blood from the placenta back to the fetus.2PubMed Central. Isolated Single Umbilical Artery and Histomorphometric Changes of Umbilical Blood Vessels in Intrauterine Growth Retarded Foetus in Comparison to Normal Neonates The naming follows the general anatomical rule that arteries move blood away from the heart and veins move blood toward it, regardless of oxygen content. But it means that the single vein is actually the lifeline delivering all of the fetus’s oxygen and nutrition, while the two arteries are the waste-disposal route.
How Three Vessels Start as Four
Early in development, the embryo actually has four umbilical vessels: two arteries and two veins. The right umbilical vein begins to shrink during the fourth week of embryonic life and disappears entirely by about the seventh week, leaving only the left umbilical vein to handle all blood returning from the placenta.3PubMed Central. The Development of the Umbilical Vein and Its Anatomical and Clinical Significance No one has a definitive explanation for why the right vein is the one that regresses rather than the left. The result, though, is reliable: by the time the cord is fully formed around week twelve, the standard configuration of two arteries and one vein is locked in.1PubMed Central. The development, structure and blood flow within the umbilical cord with particular reference to the venous system
Besides the three main blood vessels, the cord sometimes contains tiny remnants of two embryonic structures: the vitelline duct (connecting the yolk sac to the embryonic gut) and the allantoic duct. These are not blood vessels and typically regress on their own. One study of cord cross-sections at various gestational ages found that vitelline duct remnants dropped from about 11% of cords in early pregnancy to under 2% at term, while allantoic duct remnants stayed at roughly the same low prevalence regardless of gestational age.4PubMed. Changes in the Prevalence of Embryologic Remnants in Umbilical Cord With Gestational Age Neither structure carries blood or plays a role in the functioning cord by the time the baby is born.
How Wharton’s Jelly Protects the Vessels
The three vessels do not sit inside a rigid tube. They are surrounded by Wharton’s jelly, a gelatinous connective tissue unique to the umbilical cord. This material serves as a biological shock absorber. When the cord gets compressed, as it regularly does when the fetus moves or during labor, Wharton’s jelly distributes the force away from the blood vessels so they are not pinched shut. The jelly also helps the arteries function as pressure-buffering chambers during the resting phase of the fetal heartbeat, smoothing out the pulses of blood flow.5PubMed. The biomechanics of the umbilical cord Wharton Jelly: Roles in hemodynamic proficiency and resistance to compression
The cord also has a characteristic spiral, or coiling pattern. The two arteries typically wind around the vein in a helical fashion. This coiling is thought to add structural resilience, somewhat like the way a coiled telephone cord resists kinking better than a straight wire. Both the coiling and the jelly work together to keep the vessels open and blood flowing even as the fetus somersaults and the cord gets tugged and twisted.
When a Vessel Is Missing
The most common variation from the standard three-vessel cord is a single umbilical artery, where only one artery develops instead of two. In a large retrospective review of over 16,500 singleton pregnancies, 93 fetuses were found to have an isolated single umbilical artery, which puts the rate at roughly one in every 180 births in that population.6PubMed Central. Determination of risk factors and perinatal outcomes of singleton pregnancies complicated by isolated single umbilical artery in Turkish population Published estimates in the broader literature range from about 0.5% to 1% of pregnancies, depending on the population studied and how the diagnosis is made.
A two-vessel cord is usually spotted on a routine mid-pregnancy ultrasound, where the sonographer checks the cross-section of the cord near the fetal bladder. When it is an isolated finding, meaning no other structural or chromosomal abnormalities are present, outcomes are generally reassuring. However, a single umbilical artery can sometimes accompany other congenital issues, so most providers will recommend a detailed anatomy scan and potentially additional monitoring. One study of 125 growth-restricted placentas found that about 10% of them had a single umbilical artery, suggesting the condition can be associated with restricted fetal growth in some cases.2PubMed Central. Isolated Single Umbilical Artery and Histomorphometric Changes of Umbilical Blood Vessels in Intrauterine Growth Retarded Foetus in Comparison to Normal Neonates
When There Are Extra Vessels
Rarer than a missing artery is having an extra vessel. A four-vessel umbilical cord occurs when the right umbilical vein, which normally regresses in early embryonic life, persists instead of disappearing. This gives the cord two veins and two arteries. The condition is rare, and while it can be an isolated, benign finding, four-vessel cords are known to be associated with a range of congenital anomalies including heart defects, cleft lip or palate, and abdominal wall defects.7PubMed Central. Four-Vessel Umbilical Cord: Supernumerary Right Umbilical Vein With No Associated Congenital Anomalies
A related but subtly different condition is persistent right umbilical vein, or PRUV. In PRUV, the right umbilical vein survives while the left one regresses, the mirror image of normal development. The cord still has three vessels (two arteries and one vein), but the single vein is the right one instead of the left. A prospective study found PRUV in about 0.5% of pregnancies screened. In three-quarters of those cases, it was an isolated finding with healthy babies delivered at or near term.8PubMed Central. Prenatal diagnosis of persistent right umbilical vein – Incidence and clinical impact. A prospective study When PRUV and single umbilical artery appear together, though, the risk of associated vascular anomalies goes up.9PubMed Central. Persistent Right Umbilical Vein in Association With Single Umbilical Artery: A Case Report and Review of Literature
How Umbilical Vessel Walls Differ From Normal Blood Vessels
Under a microscope, umbilical cord vessels look quite different from the arteries and veins elsewhere in your body. Both the arteries and the vein are lined with endothelium, as expected. But the internal elastic layer that gives most arteries their springiness is frequently interrupted in umbilical vessels. Neither the arteries nor the vein has an external elastic layer or a distinct outer layer of connective tissue (adventitia), which ordinary blood vessels rely on for structural support.10PubMed. Histopathology and ultrastructure of human umbilical blood vessels The artery walls are thicker than the vein wall, with more layers of smooth muscle, which makes sense given that they need to push blood against resistance toward the placenta.
The absence of the usual elastic layers means these vessels do not behave like typical arteries and veins. They lack the nerve supply that controls dilation and constriction in normal blood vessels. Instead, their tone appears to be regulated locally by chemicals in the blood and by the physical properties of Wharton’s jelly pressing around them. This unusual setup is perfectly suited to the cord’s temporary job: it only needs to function for nine months, and it needs to shut down quickly at birth.
How Doctors Check Cord Blood Flow During Pregnancy
Doppler ultrasound gives clinicians a way to evaluate blood flow through the umbilical arteries without touching the cord. The technique measures the speed of blood moving through the vessels at different points in the heartbeat cycle, producing a characteristic waveform. The ratio between peak flow (during the heart’s contraction) and lowest flow (during the heart’s relaxation) serves as an indirect measure of resistance in the placental blood vessels. When that resistance is higher than expected for the gestational age, it signals placental dysfunction and potential fetal distress.11PubMed Central. Doppler Ultrasound of the Umbilical Artery: Clinical Application
In high-risk pregnancies, abnormal Doppler readings can be one of the earliest warning signs. Research comparing Doppler flow patterns with placental tissue under a microscope found that fetuses with high resistance readings had far fewer small arterial vessels in the placental villi, confirming that the Doppler waveform reflects a real structural problem in the placenta, not just a measurement artifact.12PubMed. Fetal umbilical artery flow velocity waveforms and placental resistance: pathological correlation In the most concerning cases, blood flow during the heart’s relaxation phase can slow dramatically, stop, or even reverse direction. Serial measurements showing worsening resistance over time, the opposite of the normal trend toward lower resistance as pregnancy progresses, often prompt early delivery.13PubMed. Fetal umbilical artery flow velocity waveforms and placental resistance: clinical significance
What Happens to the Vessels After Birth
Within minutes of delivery, the umbilical arteries clamp themselves shut through a remarkable self-sealing mechanism. A study examining 30 cords from normal full-term births found that multiple constrictions appeared along the arteries immediately after birth, growing in number and tightness over the first few minutes. Between these pinch points, dilated segments trapped pools of blood. When researchers cut through the constrictions in cords that had been left for more than a minute after birth, no blood leaked from the adjacent dilated sections, even though the trapped blood was still unclotted. The closure was mechanical, not dependent on clot formation.14European Journal of Obstetrics & Gynecology and Reproductive Biology. Closure of the human umbilical artery: a physiological demonstration of Burton’s theory This is why the cord can be clamped and cut safely: the arteries are already sealing on their own.
After birth, the umbilical vessels do not simply vanish. They transform into ligaments. The single umbilical vein becomes the ligamentum teres (round ligament) of the liver, a fibrous cord running along the underside of the liver to the left branch of the portal vein, tracing the path the vein followed during fetal life.15PubMed. Right ligamentum teres joining to the right branch of the portal vein The two umbilical arteries become the medial umbilical ligaments, fibrous strands running along the inner surface of the abdominal wall toward the bladder. Surgeons occasionally encounter these remnants during abdominal procedures, and they serve as useful landmarks.
Where the Cord Meets the Placenta
The three vessels of the cord normally insert into the central portion of the placenta’s fetal surface, fanning out into smaller branches that penetrate the placental tissue. In some pregnancies, the insertion point is off-center. A marginal insertion, where the cord attaches near the placental edge, occurs in about 6% of singleton pregnancies. A velamentous insertion, where the vessels travel through the fetal membranes before reaching the placenta, unprotected by Wharton’s jelly for part of their journey, is found in roughly 1.5% of singletons. Both rates are higher in twin pregnancies. A large population-based study of over 600,000 pregnancies found that velamentous insertion was associated with a tripled risk of perinatal death at term compared to normal insertion.16PubMed Central. Prevalence, Risk Factors and Outcomes of Velamentous and Marginal Cord Insertions: A Population-Based Study of 634,741 Pregnancies The danger comes from those exposed vessels being vulnerable to compression or tearing, especially during labor. Routine ultrasound can often identify abnormal insertion sites before delivery, allowing providers to plan accordingly.
Umbilical Vessel Counts Across Other Mammals
Humans are not the only species to lose a vein during development. In horses, carnivores like dogs and cats, guinea pigs, and chinchillas, the right umbilical vein also disappears, producing a three-vessel cord similar to ours. But many other mammals keep all four vessels throughout pregnancy. Cattle, sheep, goats, pigs, buffalo, camels, and even African lions and gazelles all have umbilical cords with two arteries and two veins.17Anatomia, Histologia, Embryologia. Histology of Umbilical Cord in Mammals At the other extreme, some small rodents get by with just two vessels. A recent comparative analysis across mammalian species found that species with four-vessel cords tend to give birth to larger offspring than those with three-vessel cords, while two-vessel cords were observed only in very small-bodied rodents.18bioRxiv. Umbilical cord structure shapes feto-maternal heat exchange across mammals
Why certain species keep the extra vein while others discard it remains an open question. The pattern roughly tracks body size and metabolic demands, but there are enough exceptions that no single explanation covers all cases. What is clear is that the “standard” three-vessel cord we learn about in human anatomy is just one solution among several across the mammalian family tree.