The umbilical cord connects the fetus to the placenta, facilitating nutrient and waste exchange. Typically, the cord contains three blood vessels: two arteries and one vein. A single umbilical artery (SUA) is a variation where only one artery is present, often called a two-vessel cord. SUA occurs in about 0.5% to 1% of singleton pregnancies, making it the most common umbilical cord abnormality. Although the finding can cause concern, most babies with an SUA are otherwise healthy, and the condition is often isolated with no major consequences.
The Anatomy of a Single Umbilical Artery
The standard three-vessel system includes one large vein and two smaller arteries. The vein carries oxygen-rich blood and nutrients from the placenta to the fetus. Conversely, the two umbilical arteries transport deoxygenated blood and metabolic waste away from the fetus and back to the placenta. In SUA, one of the two arteries is absent, resulting in a cord with only a single vein and a single artery. The remaining artery often compensates for the missing vessel by becoming larger to handle the entire load of deoxygenated blood flow.
The cause of this structural difference is not completely understood. Current theories suggest it may be due to the failure of one artery to develop early in pregnancy or the atrophy of a previously formed artery.
Identifying Single Umbilical Artery During Pregnancy
The detection of a single umbilical artery most commonly occurs during the routine mid-pregnancy anatomy ultrasound, typically performed between 18 and 22 weeks of gestation. The presence of only two vessels instead of the expected three alerts the clinician to the SUA diagnosis. Color Doppler imaging is frequently used to confirm the finding by visually highlighting the blood flow within the vessels. When examining the fetal pelvis, the absence of one artery on the side of the bladder is another strong indicator of SUA. While prenatal ultrasound is highly effective, the two-vessel cord is only definitively confirmed after birth through a physical examination of the placenta and umbilical cord if the finding was missed earlier.
Potential Associated Conditions
A single umbilical artery is an isolated finding in about 75% of cases, but its presence can sometimes be a soft marker for other conditions, necessitating further detailed evaluation. One concern is Fetal Growth Restriction (FGR), where the baby does not grow to its full potential. The reduced capacity of the single artery to effectively remove waste products may contribute to a smaller baby, though FGR develops in only a small percentage of isolated SUA cases.
SUA has been linked to an increased risk of anomalies in other organ systems, particularly those that develop concurrently with the umbilical arteries. Renal and urinary tract anomalies are common associated findings, as the developing umbilical arteries and kidneys share developmental pathways. These can range from minor issues like vesicoureteric reflux to more serious conditions like renal agenesis.
Cardiovascular abnormalities are also a significant association, prompting a more detailed assessment of the fetal heart. In rare instances, SUA can occur as part of a larger pattern of malformations associated with chromosomal or genetic syndromes. If other structural anomalies are found alongside the SUA, further testing, such as non-invasive prenatal testing or amniocentesis, may be offered to rule out conditions like Trisomy 18.
Monitoring and Postnatal Care
Once a single umbilical artery is diagnosed, the management plan shifts to close observation to ensure the baby is thriving. Prenatal monitoring typically involves serial growth ultrasounds, usually starting around 28 weeks, to monitor for Fetal Growth Restriction. Doppler studies are often performed alongside ultrasounds to assess the efficiency of blood flow through the single umbilical artery. A detailed fetal echocardiogram may also be recommended to provide a specialized view of the baby’s heart structure and function, especially if the initial anatomy scan raised concerns.
The postnatal care protocol depends on whether the SUA was isolated or if associated anomalies were identified during pregnancy. For an isolated SUA, the newborn receives routine care, though a thorough physical examination focused on the heart and kidneys is standard practice. If the prenatal evaluation suggested associated issues, an abdominal ultrasound of the baby’s kidneys and urinary tract is often performed shortly after birth. A consultation with a pediatric cardiologist may also be arranged if concerns arose during the fetal echocardiogram.