The umbilical cord is a temporary structure that serves as the biological connection between a developing fetus and the placenta. This cord is the primary conduit for gas and nutrient exchange during gestation. It forms early in embryological development, ensuring a continuous pathway for blood flow that allows the fetus to grow within the uterine environment.
The Typical Structure of the Umbilical Cord
The standard umbilical cord contains three distinct blood vessels, a configuration found in the majority of pregnancies. This structure is composed of two smaller umbilical arteries and one larger umbilical vein. These three vessels are bundled together and encased within a specialized, protective substance.
This surrounding material is known as Wharton’s Jelly, a type of gelatinous connective tissue. Wharton’s Jelly provides cushioning and structural resilience, preventing the vessels from becoming compressed or kinked during fetal movement. The vessels and the jelly are then enclosed by an outer layer of amnion, which is continuous with the covering of the placenta.
Roles of the Arteries and Vein
The function of the vessels within the umbilical cord is unique because their roles are reversed compared to adult circulation. The single umbilical vein carries blood enriched with oxygen and nutrients directly from the placenta to the fetus. This flow is necessary because the fetal lungs are not yet functional for gas exchange.
The two umbilical arteries serve the opposite purpose, transporting used blood away from the fetus and back to the placenta. This blood is deoxygenated and nutrient-depleted, carrying metabolic waste products and carbon dioxide from the fetal circulation. The placenta then transfers these waste products to the maternal circulation for disposal.
This bidirectional flow ensures the fetus receives a continuous supply of oxygen and glucose while simultaneously removing waste. The arteries originate from the internal iliac arteries of the fetus, emphasizing their role in returning blood to the placental exchange site. This specific circulatory pattern is a temporary adaptation that closes shortly after birth when the newborn begins breathing air.
Understanding Single Umbilical Artery
While the standard structure involves three vessels, a variation known as Single Umbilical Artery (SUA) is the most frequently observed vascular anomaly. In this condition, the cord contains only two vessels: one artery and one vein. The prevalence of SUA is estimated to be between 0.5% and 6% of all pregnancies worldwide.
The absence of one artery can sometimes be an isolated finding, where the fetus appears otherwise healthy and develops normally. However, SUA is considered a marker that warrants further investigation due to its link with an increased risk of congenital anomalies. These anomalies most commonly involve the cardiovascular, genitourinary, and gastrointestinal systems.
If SUA is identified during a prenatal ultrasound, physicians may recommend a more detailed assessment, such as a fetal echocardiogram, to rule out heart defects. When SUA is found alongside other structural issues, the risk for chromosomal anomalies is also increased. Monitoring fetal growth is standard practice, as SUA is associated with an increased risk of fetal growth restriction and low birth weight.