The umbilical cord serves as an important connection between a developing fetus and the placenta, facilitating the exchange of nutrients, oxygen, and waste products. Typically, this cord contains three blood vessels: one vein and two arteries. However, a variation known as a two-vessel cord, or Single Umbilical Artery (SUA), occurs when only one umbilical artery is present alongside the single vein. This condition is recognized as the most common umbilical abnormality identified during pregnancy.
Understanding the Umbilical Cord and SUA
A normal umbilical cord transports blood, with the single umbilical vein delivering oxygenated, nutrient-rich blood from the placenta to the fetus. The two umbilical arteries carry deoxygenated blood and waste products back to the placenta from the fetus. These vessels are encased in a protective substance called Wharton’s jelly, which helps prevent compression.
A two-vessel cord means one umbilical artery is absent, leaving only one artery and one vein. The prevalence of SUA ranges from approximately 0.4% to 1% in singleton pregnancies, and it is observed more frequently in multiple pregnancies. Often, SUA is an isolated finding, meaning no other anomalies are detected, and babies are born healthy.
Unraveling the Causes of SUA
The cause of a single umbilical artery is often unknown, suggesting multiple factors. One theory suggests it arises from a developmental accident where an umbilical artery fails to form or atrophies early in gestation. This can involve initial formation of three vessels, followed by regression or blockage of one artery.
While genetic factors are not the primary cause for most isolated SUA cases, some rare chromosomal abnormalities and genetic syndromes can be associated. For instance, certain trisomies (e.g., Trisomy 18) have a higher association. Maternal conditions like diabetes and hypertension, and certain environmental exposures, have also been observed. However, these are considered associations rather than direct causes, as SUA can occur without them.
Associated Conditions and Monitoring
While many SUA cases are isolated and benign, its presence can indicate an increased risk for other anomalies. Fetuses with SUA have a slightly elevated risk of chromosomal abnormalities (around 5% of cases), particularly Trisomy 18, 13, and 21. However, these conditions are usually accompanied by other detectable structural abnormalities.
Cardiac defects are a concern, as the heart and umbilical vessels share developmental pathways. Between 6.5% and 11.4% of fetuses with SUA may have associated cardiac anomalies, such as ventricular septal defects. Renal or urinary tract anomalies are also more prevalent (about 7-9% of isolated cases).
Fetal growth restriction (baby smaller than average) is another concern, occurring in about 10-15% of SUA pregnancies. Once SUA is identified (usually via routine second-trimester ultrasound), a comprehensive monitoring protocol is recommended. This includes a detailed ultrasound to screen for other structural anomalies and a fetal echocardiogram to assess the fetal heart. Serial growth scans are also performed to monitor fetal development and detect growth restriction.
Outlook and Management
For most pregnancies where SUA is an isolated finding (no other abnormalities detected after evaluation), the outlook is positive. Most babies born with isolated SUA are healthy and develop normally. Isolated SUA does not typically increase the risk of adverse perinatal outcomes like prematurity or low birth weight in uncomplicated singleton pregnancies.
Management for isolated SUA primarily involves close observation, not active intervention for the cord itself. Healthcare providers monitor fetal growth and well-being through regular ultrasound scans, especially in the third trimester. This approach helps ensure potential complications, though rare, are identified and addressed promptly. Communication with medical professionals is important for parents to understand the individualized monitoring plan and address concerns.