The umbilical cord connects a developing fetus to the placenta throughout pregnancy. This circulatory structure exchanges oxygen, nutrients, and waste products between the maternal blood supply and the fetus. To facilitate this process, the human umbilical cord typically contains three distinct blood vessels.
The Standard Anatomical Configuration
The typical umbilical cord contains three blood vessels: two arteries and one vein. These vessels are responsible for maintaining the continuous flow of blood necessary to support fetal growth and development. The two umbilical arteries are smaller and carry blood away from the fetus toward the placenta.
This directional flow is a unique feature of fetal circulation that reverses the function typically associated with arteries in an adult. The umbilical arteries carry blood that is depleted of oxygen and nutrients, along with metabolic waste products, from the fetal system to the placenta for disposal. These arteries are actually extensions of the fetal internal iliac arteries.
The single umbilical vein is larger than the two arteries and is responsible for transporting oxygenated blood back to the fetus from the placenta. This blood is rich in oxygen and vital nutrients absorbed from the maternal circulation across the placental barrier. Therefore, in the umbilical cord, the vein carries the highest concentration of oxygen, which is the opposite of a vein’s role in the postnatal body. Both the arteries and the vein spiral around each other within the cord, a natural twist that adds strength and flexibility to the entire structure.
The Protective Structure of the Umbilical Cord
The blood vessels within the umbilical cord are encased and protected by a specialized substance known as Wharton’s Jelly. This gelatinous material is a type of mucoid connective tissue, providing structural integrity and cushioning to the entire cord. Its composition is primarily a network of mucopolysaccharides, such as hyaluronic acid and chondroitin sulfate, which create a firm yet flexible matrix.
The protective function of Wharton’s Jelly is crucial for maintaining uninterrupted blood flow to the fetus. The firm, rubbery nature of the jelly acts as a shock absorber, preventing the vessels from being compressed, kinked, or otherwise damaged by external pressure or fetal movement. This cushioning ensures that the arteries and vein can continuously transport blood without obstruction. The entire structure of the cord is further enclosed by a thin, protective layer derived from the amnion.
When the Vessel Count is Different
While the standard is a three-vessel cord, the most common anatomical variation is the presence of only two vessels: one artery and one vein. This condition is known as Single Umbilical Artery (SUA). The prevalence of SUA is estimated to occur in approximately 0.5% to 1% of all singleton pregnancies.
The precise cause of SUA is often unknown, though it may result from the early atrophy or failure of one of the two standard umbilical arteries to develop. In many instances, the finding of a two-vessel cord is an isolated one, meaning the fetus has no other associated health issues, and the pregnancy outcome is typically normal. Approximately 75% of fetuses with an SUA are otherwise healthy and grow appropriately.
However, the presence of SUA is considered a soft marker for other anomalies, necessitating a more detailed ultrasound examination. The remaining cases are associated with an increased risk of structural abnormalities, particularly involving the cardiovascular, urinary, and skeletal systems. A diagnosis of SUA often prompts an intensive evaluation of fetal anatomy, known as a detailed sonoanatomy survey.
The diagnosis also warrants closer monitoring for potential complications such as fetal growth restriction. If additional anomalies are detected alongside the SUA, the risk of chromosomal abnormalities increases significantly. When the SUA is confirmed to be an isolated finding after a thorough screening, the prognosis is generally favorable, but closer surveillance of fetal growth may be recommended.