Umbilical Arteries

The umbilical arteries are a pair of fetal blood vessels that carry blood from the fetus to the placenta. They are major components of the fetoplacental circulation and travel through the umbilical cord alongside the single umbilical vein.

Unlike most arteries in the postnatal circulation, the umbilical arteries carry relatively deoxygenated blood. This occurs because arteries are defined by the direction in which they carry blood relative to the heart, not by the oxygen content of that blood. The umbilical arteries carry blood away from the fetal heart and toward the placenta.

Within the placenta, their branches participate in an extensive fetal capillary network where exchange occurs between fetal and maternal circulations. After birth, most of each umbilical artery closes and forms a medial umbilical ligament, while the proximal portions remain patent as arteries supplying the urinary bladder and nearby pelvic structures.

Number of Umbilical Arteries

The normal umbilical cord contains two umbilical arteries and one umbilical vein.

The paired arteries transport fetal blood toward the placenta, while the single umbilical vein returns oxygenated and nutrient-rich blood from the placenta to the fetus.

Origin

During fetal life, the umbilical arteries arise from the pelvic arterial circulation. They are continuous with branches of the internal iliac arteries.

From the pelvis, each artery travels toward the anterior abdominal wall and umbilicus before entering the umbilical cord.

Relationship to the Internal Iliac Arteries

The internal iliac arteries supply the pelvic region and give rise to the fetal umbilical arteries.

After birth, the proximal patent portions of the umbilical arteries remain associated with the internal iliac arterial system.

Course in the Pelvis

Each umbilical artery courses anteriorly within the pelvis toward the urinary bladder.

The vessels then ascend on either side of the bladder toward the umbilicus.

Relationship to the Urinary Bladder

The umbilical arteries are closely related to the superior and lateral aspects of the fetal urinary bladder.

This relationship remains evident after birth because the obliterated distal portions form fibrous cords that ascend from the pelvis toward the umbilicus.

Course Toward the Umbilicus

After passing along the bladder, the arteries ascend on the deep surface of the anterior abdominal wall.

They converge toward the umbilical region and enter the umbilical cord.

Course Within the Umbilical Cord

Within the umbilical cord, the two umbilical arteries travel alongside the single umbilical vein.

The vessels are embedded in a specialized gelatinous connective tissue called Wharton's jelly.

Wharton's Jelly

Wharton's jelly surrounds and supports the umbilical vessels.

Its hydrated connective tissue helps protect the vessels against compression, bending, and torsion as the fetus moves and the cord changes position.

Umbilical Cord Vessel Arrangement

Vessel	Number	Direction of Blood Flow	Relative Oxygen Content
Umbilical arteries	Two	Fetus → Placenta	Relatively deoxygenated
Umbilical vein	One	Placenta → Fetus	Relatively oxygenated

Function

The primary function of the umbilical arteries is to carry fetal blood to the placenta for exchange with the maternal circulation.

This blood contains carbon dioxide and metabolic waste products that can be transferred across the placental barrier while oxygen and nutrients are transferred toward the fetal circulation.

Fetoplacental Circulation

The umbilical arteries form the outgoing vascular component of the fetoplacental circulation.

Blood leaves the fetal systemic arterial circulation, enters the umbilical arteries, travels through the umbilical cord, and reaches the placenta.

Placental Branching

At the placenta, the umbilical arteries divide into branches that spread across the fetal surface of the placenta.

These vessels branch progressively into smaller arteries and arterioles associated with the chorionic villi.

Chorionic Villi

The chorionic villi contain fetal blood vessels and project into spaces containing maternal blood.

The fetal and maternal blood normally remain within separate circulatory compartments, while gases, nutrients, waste products, and other substances cross the placental barrier.

Placental Capillary Network

Small branches of the umbilical arteries ultimately supply capillary networks within the chorionic villi.

After exchange occurs, fetal blood is collected into venous channels that ultimately form the umbilical vein.

Direction of Blood Flow

The major fetal placental pathway can be represented as:

Fetal systemic circulation → Internal iliac arteries → Umbilical arteries → Placenta → Umbilical vein → Fetus

Why Umbilical Arteries Carry Deoxygenated Blood

The term artery refers to a vessel carrying blood away from the heart.

Because the umbilical arteries carry blood away from the fetal heart toward the placenta, they are arteries even though their blood has a lower oxygen content than blood returning through the umbilical vein.

Comparison With Adult Systemic Arteries

Feature	Umbilical Arteries	Typical Adult Systemic Arteries
Direction	Away from fetal heart	Away from heart
Destination	Placenta	Body tissues
Relative oxygen content	Lower	Usually higher
Main role	Carry blood for placental exchange	Deliver oxygenated blood to tissues

Relationship to the Umbilical Vein

The two umbilical arteries and single umbilical vein form the major vascular structures of the umbilical cord.

The arteries carry blood toward the placenta, while the vein carries blood in the opposite direction toward the fetus.

Relationship to the Placenta

The placenta functions as the fetal organ of gas, nutrient, and waste exchange.

The umbilical arteries deliver blood to the fetal vascular component of the placenta, where it approaches maternal blood across the placental exchange barrier.

Maternal and Fetal Blood

Maternal and fetal blood normally do not flow directly into one another.

Maternal blood surrounds the chorionic villi within the intervillous space, while fetal blood remains within capillaries inside the villi.

Placental Gas Exchange

Oxygen moves from maternal blood toward fetal blood, while carbon dioxide moves in the opposite direction.

After becoming more oxygenated within the placenta, fetal blood returns through the umbilical vein.

Placental Nutrient Exchange

Glucose, amino acids, electrolytes, and other nutrients are transferred across the placental barrier by several transport mechanisms.

The umbilical circulation distributes these substances between the placenta and fetus.

Waste Removal

Fetal metabolic waste products are transported to the placenta through the umbilical arteries.

They cross into the maternal circulation and are subsequently processed or excreted by maternal organs.

Umbilical Arteries in the Fetal Circulatory Pathway

Blood entering the descending aorta supplies the lower fetal body and pelvic circulation.

A substantial portion is then directed through the umbilical arteries toward the placenta.

Relationship to the Ductus Arteriosus

Much of the right ventricular output in the fetus passes from the pulmonary trunk through the ductus arteriosus into the descending aorta.

This blood contributes to the systemic flow that ultimately reaches the umbilical arteries and returns to the placenta.

Relationship to the Ductus Venosus

After placental exchange, blood returns through the umbilical vein.

A substantial portion of this blood passes through the ductus venosus toward the inferior vena cava, completing another major part of the fetal circulatory pathway.

Relationship to the Foramen Ovale

Relatively oxygen-rich blood reaching the right atrium from the inferior vena cava is preferentially directed across the foramen ovale into the left atrium.

The umbilical arteries therefore participate in a circulation linked functionally with all three major fetal shunts.

Complete Fetoplacental Loop

A simplified circulation loop can be represented as:

Placenta → Umbilical vein → Fetal heart → Fetal systemic circulation → Umbilical arteries → Placenta

Embryological Development

The umbilical arteries develop in association with the embryonic arterial system and initially connect with branches related to the dorsal aortae.

As the pelvic arterial circulation develops and remodels, their definitive fetal origin becomes associated with the internal iliac arteries.

Allantoic Region

Early umbilical vessels develop in close association with the connecting stalk and allantoic region.

As the expanding amnion incorporates the connecting stalk into the umbilical cord, the umbilical vessels become enclosed within the developing cord.

Changes at Birth

Clamping and division of the umbilical cord abruptly interrupt blood flow through the umbilical vessels.

The umbilical arteries constrict rapidly, helping limit blood loss from the newborn.

Arterial Constriction

The muscular walls of the umbilical arteries are capable of strong contraction.

Following birth, vascular smooth muscle contraction contributes to rapid functional closure of the distal vessels.

Anatomical Obliteration

After functional closure, the distal intra-abdominal portions of the umbilical arteries undergo progressive fibrosis and obliteration.

They persist as fibrous structures extending from the pelvis toward the umbilicus.

Medial Umbilical Ligaments

The obliterated distal portions of the umbilical arteries become the paired medial umbilical ligaments.

These ligaments extend from the pelvic region toward the umbilicus on the deep surface of the anterior abdominal wall.

Medial Umbilical Folds

The medial umbilical ligaments elevate the overlying parietal peritoneum to form the medial umbilical folds.

These folds are visible from the internal surface of the anterior abdominal wall.

Proximal Patent Portions

The proximal portions of the umbilical arteries do not normally become completely obliterated.

They remain patent and contribute to the arterial supply of the urinary bladder.

Superior Vesical Arteries

The patent proximal segments give rise to the superior vesical arteries, which supply the superior portion of the urinary bladder and adjacent structures.

Fetal and Adult Derivatives

Fetal Structure	Postnatal Fate
Distal umbilical arteries	Medial umbilical ligaments
Proximal umbilical arteries	Remain patent and contribute to superior vesical arterial supply

Medial Versus Median Umbilical Structures

The medial umbilical ligaments should not be confused with the median umbilical ligament.

The paired medial umbilical ligaments are remnants of the umbilical arteries, while the single median umbilical ligament is the remnant of the urachus.

Umbilical Folds of the Anterior Abdominal Wall

Fold	Underlying Structure
Median umbilical fold	Median umbilical ligament, remnant of urachus
Medial umbilical folds	Obliterated umbilical arteries
Lateral umbilical folds	Inferior epigastric vessels

Clinical Importance of Umbilical Fold Anatomy

The umbilical folds are important landmarks on the internal surface of the anterior abdominal wall.

The fossae between them have relationships to common sites of abdominal wall hernias, making recognition of the folds important in anatomy and surgery.

Single Umbilical Artery

A single umbilical artery occurs when the umbilical cord contains only one artery instead of the usual pair.

It may occur as an isolated finding or in association with other fetal abnormalities.

Detection of a Single Umbilical Artery

Prenatal ultrasound can evaluate the number of vessels within the umbilical cord.

Color Doppler can also demonstrate the arteries coursing around the fetal urinary bladder.

Umbilical Artery Doppler

Umbilical artery Doppler ultrasonography measures blood-flow velocity within an umbilical artery during the cardiac cycle.

The waveform provides indirect information about resistance within the placental vascular bed.

Systolic and Diastolic Flow

Blood velocity is highest during ventricular systole, but forward flow normally continues during diastole as pregnancy progresses.

The amount of end-diastolic flow is influenced substantially by downstream placental vascular resistance.

Placental Vascular Resistance

As normal placental vascular development progresses, the extensive branching vascular network provides a relatively low-resistance circulation.

Abnormal increases in placental vascular resistance can alter the umbilical artery Doppler waveform.

Absent End-Diastolic Flow

In some forms of severe placental vascular dysfunction, forward flow during end diastole may become absent.

This is an abnormal Doppler finding that is interpreted together with gestational age, fetal growth, other Doppler studies, and the overall clinical condition.

Reversed End-Diastolic Flow

With marked increases in downstream resistance, blood may transiently move backward in the umbilical artery during diastole.

Reversed end-diastolic flow is a significant abnormal finding requiring clinical interpretation in the context of the complete fetal assessment.

Umbilical Artery Blood Sampling

After delivery, blood obtained from an umbilical artery can be analyzed to provide information about fetal acid-base status around the time of birth.

Arterial cord blood reflects fetal metabolic conditions more directly than umbilical venous blood because it is traveling from the fetus toward the placenta.

Umbilical Artery Versus Umbilical Vein Blood

Feature	Umbilical Artery	Umbilical Vein
Direction	Fetus → Placenta	Placenta → Fetus
Oxygen content	Lower	Higher
Carbon dioxide	Higher	Lower
Number in normal cord	Two	One

Umbilical Artery Catheterization

In neonatal care, an umbilical arterial catheter may be inserted through an umbilical artery shortly after birth when clinically indicated.

The catheter can provide arterial access for blood sampling and continuous blood pressure monitoring in selected newborns.

Anatomical Course of an Umbilical Arterial Catheter

A catheter introduced through an umbilical artery travels from the umbilicus inferiorly through the arterial remnant toward the internal iliac circulation and then into the common iliac artery and aorta.

Knowledge of this course is important for appropriate catheter positioning.

Umbilical Arteries and Cord Compression

Because fetal blood must pass continuously through the umbilical vessels, significant compression of the cord can alter fetoplacental blood flow.

Wharton's jelly and the coiled configuration of the cord help protect the vessels from ordinary mechanical forces.

Vessel Wall Structure

The umbilical arteries have muscular walls adapted to fetal circulation and rapid postnatal constriction.

They lack some of the structural features typical of comparable adult systemic arteries and are surrounded directly by Wharton's jelly rather than by the usual arrangement of surrounding tissues.

Key Features of the Umbilical Arteries

Feature	Key Point
Number	Normally two
Origin	Internal iliac arterial system
Direction of flow	Fetus → Placenta
Blood carried	Relatively deoxygenated fetal blood
Location in cord	Travel with one umbilical vein within Wharton's jelly
Primary function	Deliver fetal blood to placenta for exchange
Distal postnatal remnant	Medial umbilical ligaments
Proximal postnatal fate	Remains patent and contributes to superior vesical arteries

Anatomical and Physiological Importance

The umbilical arteries are essential components of fetal circulation because they provide the vascular route by which fetal blood reaches the placenta. Their unusual oxygenation pattern demonstrates an important anatomical principle: arteries are classified by the direction of blood flow away from the heart rather than by oxygen content.

Their anatomy also links fetal circulation with adult pelvic anatomy. During fetal life, they are large functional vessels extending from the internal iliac circulation to the placenta. After birth, their distal portions become the medial umbilical ligaments, while their proximal portions remain patent and continue to supply the urinary bladder.

The umbilical arteries therefore provide an important anatomical connection among fetal cardiovascular physiology, placental exchange, embryological development, the internal surface of the anterior abdominal wall, and the adult arterial supply of the pelvis.