A two-vessel cord means the umbilical cord contains one artery and one vein instead of the usual two arteries and one vein, a finding also called a single umbilical artery. It is the most common congenital abnormality of the umbilical cord, and most of the time a baby with an isolated two-vessel cord does perfectly well. But the finding does prompt extra screening, because it can occasionally travel with heart defects, kidney anomalies, or other structural problems that benefit from early detection.
What a Normal Cord Looks Like and What Changes
A typical umbilical cord has three vessels: two arteries that carry deoxygenated blood and waste from the fetus to the placenta, and one vein that carries oxygen-rich blood back. In a two-vessel cord, one of those arteries is missing. The prevailing explanation among embryologists is that the artery forms normally early in development and then shrinks or closes off, a process called secondary atresia. Two other theories exist: that the artery simply never forms in the first place, or that a persistent embryonic vessel disrupts the normal pattern. The secondary-atresia explanation is considered the most likely from an embryological standpoint.1Journal of Pathology and Translational Medicine. Single umbilical artery and associated birth defects in perinatal autopsies: prenatal diagnosis and management
Because only one artery remains to handle all the blood flow back to the placenta, that remaining artery is often wider than either artery would be in a three-vessel cord. This compensatory widening generally keeps circulation adequate, which is one reason most babies with a two-vessel cord grow and develop normally.
How Common It Is
In singleton pregnancies, a two-vessel cord shows up in roughly 0.5 to 1 percent of deliveries, making it something ultrasound technicians encounter regularly. The rate is considerably higher in twin pregnancies, reported at around 4 to 11 percent.2PubMed Central. Isolated single umbilical artery in twin pregnancies and its adverse pregnancy outcomes – a case report and review of literature In one study of twins, about 10 percent of twin pregnancies had at least one fetus with a single umbilical artery, affecting roughly 5 percent of individual twin fetuses.3PubMed. Single umbilical artery in twin pregnancies Most twin pregnancies that showed it were discordant, meaning only one twin had the two-vessel cord while the other had a normal three-vessel cord.
Maternal smoking appears to raise the odds. In one Turkish population study, smoking was independently associated with about a three-and-a-half-fold increase in the likelihood of a single umbilical artery.4PubMed Central. Determination of risk factors and perinatal outcomes of singleton pregnancies complicated by isolated single umbilical artery in Turkish population Maternal diabetes has also been linked to the condition.
How It Is Found on Ultrasound
A two-vessel cord is typically picked up during routine prenatal ultrasound. Current guidelines recommend assessing the number of cord vessels as early as the first trimester, along with evaluating where the cord inserts into the abdomen and the placenta.5PubMed Central. Modern Ultrasonography of the Umbilical Cord: Prenatal Diagnosis of Umbilical Cord Abnormalities and Assessement of Fetal Wellbeing In practice, most diagnoses happen at the 18-to-20-week anatomy scan, when the cord vessels can be seen clearly in cross-section. On a color Doppler image, you’ll see two circles instead of three where the cord enters the baby’s belly.
Once a two-vessel cord is spotted, the next question is whether anything else looks unusual. If the rest of the anatomy scan is normal, the finding is classified as “isolated,” and that distinction matters enormously for what happens next.
When Other Anomalies Are Present
A two-vessel cord can appear alongside structural birth defects. In a large retrospective review of over 12,000 placentas, roughly half of the cases with a two-vessel cord had other fetal anomalies. The most common associations were cardiac defects, neural tube defects, and genitourinary problems.6American Journal of Clinical Pathology. The Incidence of Two-Vessel Umbilical Cords and its Association With Other Fetal Anomalies: A Retrospective Study of Over 12,000 Placentas In an earlier ultrasound-based study, 82 fetuses were diagnosed with a two-vessel cord prenatally. Ten turned out to have chromosomal abnormalities, nine of whom had visible structural defects. Among the 72 with normal chromosomes, 31 had other anomalies found after birth, and most of those had already been flagged on ultrasound.7PubMed. Prenatal diagnosis of the two-vessel cord: implications for patient counselling and obstetric management
These numbers sound alarming, but there’s a catch: studies that draw from pathology databases or referral centers over-represent complicated cases. Babies whose cords are sent for pathology review tend to be the ones where something else was already going wrong. When you look at unselected populations, meaning all pregnancies rather than only the worrisome ones, the picture is much more reassuring, especially when the two-vessel cord is isolated.
The Cardiac Question
Heart defects are the association that gets the most attention, and there is genuine debate about how aggressively to screen for them. One institution in Cleveland adopted a policy of mandatory referral to pediatric cardiology for every fetus diagnosed with a single umbilical artery, regardless of whether the anatomy scan suggested a heart problem. Their reasoning was that the fetal heart is complex and difficult to evaluate fully at 18 to 20 weeks.8PubMed Central. Prenatal Detection of Cardiac Anomalies in Fetuses with Single Umbilical Artery: Diagnostic Accuracy Comparison of Maternal-Fetal-Medicine and Pediatric Cardiologist
Their data showed that when the maternal-fetal-medicine physician found an isolated single umbilical artery and no other abnormalities, the risk of completely missing a significant heart defect was about 1 percent. That is reassuringly low, but the same study found that some cardiac and vascular abnormalities were only discovered after birth, on neonatal echocardiography, even when prenatal specialists had looked.9PubMed Central. Prenatal Detection of Cardiac Anomalies in Fetuses with Single Umbilical Artery: Diagnostic Accuracy Comparison of Maternal-Fetal-Medicine and Pediatric Cardiologist
A separate analysis across both selected and unselected populations found that the link between a single umbilical artery and heart defects depends heavily on how the study population is assembled. In an unselected population where the fetus otherwise appears normal, the added risk of congenital heart disease seems small and may not justify specialist fetal echocardiography.10PubMed. Single umbilical artery and congenital heart disease in selected and unselected populations In practice, many hospitals still recommend at least a detailed look at the heart during the anatomy scan when a two-vessel cord is found, but universal referral to a pediatric cardiologist is not standard everywhere.
Kidney and Urinary Tract Screening
The kidneys and the umbilical arteries share an embryological origin, which is why kidney anomalies are another well-known association. In a study of 271 newborns with a single umbilical artery who underwent kidney ultrasound, about 7 percent had a congenital abnormality of the kidneys or urinary tract. The most common finding was hydronephrosis, which is swelling of the kidney from urine backing up, followed by missing one kidney entirely and horseshoe kidney.11PubMed Central. Risks of Renal Anomalies and Urinary Tract Infections in Neonates With Single Umbilical Artery
Because of this link, many providers order a postnatal kidney ultrasound for newborns with a two-vessel cord. The screening is painless and low-cost, and catching something like vesicoureteral reflux (where urine flows backward from the bladder toward the kidneys) early can prevent recurrent urinary tract infections down the road. Not every guideline mandates this, but it is common practice in most high-income settings.
Chromosomal Risk and Genetic Testing
Parents often worry that a two-vessel cord signals a chromosomal problem like Down syndrome. Here, the evidence is strongly reassuring when the finding is isolated. In one study of over 12,000 pregnancies, fetuses with a two-vessel cord and no other abnormalities had about a 2.5 percent rate of chromosomal problems. But fetuses with a two-vessel cord plus major structural anomalies had a dramatically higher rate, around 42 percent.12Ultrasound Quarterly. The Value of Single Umbilical Artery in the Prediction of Fetal Aneuploidy: Findings in 12,672 Pregnant Women A Slovak population study came to an even firmer conclusion: every chromosomally abnormal fetus in their series also had associated congenital anomalies, meaning none had a completely isolated single umbilical artery.13PubMed. Single umbilical artery and reproduction losses in Slovak population: relation to karyotype and fetal anomalies
The Society for Maternal-Fetal Medicine puts it plainly: for patients with an isolated single umbilical artery, there appears to be no increased risk of aneuploidy. Their recommendation is that no additional genetic evaluation is needed in these cases, whether or not previous screening was performed.14American Journal of Obstetrics and Gynecology. SMFM Consult Series #57: Evaluation and management of isolated soft ultrasound markers for aneuploidy in the second trimester This is important: a two-vessel cord by itself is not a reason to push for amniocentesis or cell-free DNA testing beyond whatever screening was already planned.
Growth and Pregnancy Complications
Even when a two-vessel cord is isolated, pregnancy outcomes are slightly different from those with a normal cord. A large study from the Middle East found that pregnancies with an isolated single umbilical artery were more likely to involve preterm delivery before 34 weeks, birth weight below the 10th percentile, cesarean delivery for fetal distress, and admission to the neonatal intensive care unit. There was also an increased rate of placental abnormalities and polyhydramnios (excess amniotic fluid).15PubMed. Outcomes of an isolated single umbilical artery in singleton pregnancy: a large study from the Middle East and Gulf region
A population-based Scandinavian study echoed these findings, reporting an increased risk of preterm birth, small-for-gestational-age babies, small placentas, abnormal cord insertion, and complications during the third stage of labor (delivering the placenta).16PubMed. Isolated single umbilical artery and the risk of adverse perinatal outcome and third stage of labor complications: A population-based study
Because of the higher likelihood of growth restriction, providers often monitor fetal growth more closely in these pregnancies. Researchers have proposed using different reference charts for Doppler blood-flow measurements in the umbilical artery when a two-vessel cord is present. The single remaining artery has different normal flow patterns than each artery in a three-vessel cord, and using standard reference values could cause clinicians to miss early signs of growth restriction.17PubMed. Reference charts for umbilical Doppler pulsatility index in fetuses with isolated two-vessel cord This is a technical detail, but the practical takeaway is that your provider should be aware the cord is two-vessel when interpreting Doppler studies later in pregnancy.
Does It Matter Which Artery Is Missing?
The umbilical arteries are not identical; they branch from different sides of the fetal circulation, and studies have asked whether one side being absent carries different risks than the other. The left artery is absent more often in most series. One early study found the left artery was missing in about 73 percent of cases compared to 27 percent for the right, and all complex congenital anomalies and all chromosomal abnormalities in their cohort occurred in fetuses missing the left artery.18American Journal of Obstetrics and Gynecology. Single umbilical artery: Does it matter which artery is missing?
However, more recent and larger studies complicate that picture. A Chinese retrospective study of nearly a thousand cases found the left and right arteries absent at similar rates overall, but when anomalies were present, the right artery was absent more often.19PubMed Central. Diagnosis of single umbilical artery and risk of foetal congenital malformations by prenatal ultrasound: a retrospective study And a multivariate analysis focused specifically on gastrointestinal abnormalities found a roughly threefold increase in GI anomalies when the right artery was absent compared to the left.20PubMed Central. Single Umbilical Artery: Does Side Matter?
In short, the evidence on laterality is mixed. Some patterns have emerged—gastrointestinal issues may track with a missing right artery—but the findings are not consistent enough to drive different management depending on which side is absent. Your provider may note the laterality and factor it into the overall picture, but it won’t fundamentally change the screening approach.
Long-Term Outcomes for Babies With an Isolated Two-Vessel Cord
This is the question most parents really want answered after the initial anxiety settles: will my child be fine down the road? A study that followed children born with an isolated single umbilical artery through early childhood compared their physical growth and neurological development against children with normal cords. After controlling for other factors, there were no clinically meaningful differences in height, head circumference, intelligence testing, or neurological milestones. The only statistically significant difference was in body weight, and even that appeared only at birth, four months, and seven years, with differences so small—a fraction of a kilogram at each point—that they had no practical significance.21PubMed Central. Long-term physical and neurologic development in newborn infants with isolated single umbilical artery
This is genuinely good news for the vast majority of families. When the two-vessel cord is the only finding and the baby arrives without other complications, the long-term trajectory looks essentially the same as for any other child.
What Monitoring Looks Like in Practice
If you’ve been told your baby has a two-vessel cord, the practical management typically unfolds in a predictable pattern. First, the anatomy scan is reviewed carefully, or sometimes repeated with a more experienced specialist, to check for structural anomalies in the heart, kidneys, brain, and elsewhere. If everything else looks normal, the finding is classified as isolated.
From there, most providers will add periodic growth ultrasounds in the third trimester to watch for growth restriction. The exact schedule varies by practice, but every three to four weeks in the final trimester is common. Some will also order Doppler flow studies of the umbilical artery, keeping in mind that the single-artery flow pattern has its own norms.
After delivery, many institutions order a postnatal kidney ultrasound within the first few months. As noted earlier, about 7 percent of these babies have some kidney or urinary tract finding, and early detection allows proactive management of conditions that could otherwise present as unexplained urinary infections in infancy.22PubMed Central. Risks of Renal Anomalies and Urinary Tract Infections in Neonates With Single Umbilical Artery Beyond these checks, no ongoing surveillance is needed for an otherwise healthy baby.
Cord Insertion and Placental Findings
A two-vessel cord sometimes travels with unusual placental features. Marginal cord insertion (where the cord attaches near the edge of the placenta rather than centrally) and velamentous insertion (where the cord vessels run through the membranes before reaching the placenta) are both seen more often alongside a single umbilical artery. Population-level data confirm higher rates of small placentas, placenta previa, and abruption in these pregnancies.23PubMed. Isolated single umbilical artery and the risk of adverse perinatal outcome and third stage of labor complications: A population-based study
The same study noted a higher rate of third-stage complications, meaning problems delivering the placenta after the baby is born. Providers attending a delivery with a known two-vessel cord are typically aware of this and prepared to manage a retained placenta or unusual bleeding. For the parent, this is not something you need to worry about actively—it is handled in the delivery room—but it is worth knowing about so the extra attention from the medical team after delivery does not come as a surprise.
When It Shows Up in Twins
Twin pregnancies deserve their own mention because the rate of single umbilical artery is meaningfully higher. In one cohort, about 10 percent of twin pregnancies were affected, with no significant difference between identical and fraternal twins.24PubMed. Single umbilical artery in twin pregnancies When one twin has a two-vessel cord and the other does not, the discordance can complicate growth comparisons, since one twin may be slightly smaller for reasons related to the cord rather than anything more worrying. Providers managing twin pregnancies with a discordant two-vessel cord generally increase the frequency of growth scans and watch for widening size differences between the twins.
The management principles are the same as in singletons—screen carefully for anomalies, monitor growth, and expect a good outcome when the finding is isolated—but the baseline complexity of twin pregnancies means the overall surveillance tends to be more intensive regardless.