Anatomy, Abdomen and Pelvis: Umbilical Cord

Structure and Function

Anatomical Features of the Umbilical Cord

The umbilical cord appears as a soft, tortuous structure with a smooth outer covering of amnion. Extension spans from the fetal umbilicus to the placental center. Variability in placental attachment gives rise to differences in insertion. Average length ranges from 50 to 60 cm, with a diameter of approximately 1 cm.[6] Within the cord lies a gelatinous connective tissue known as the Wharton jelly (substantia gelatinea funiculi umbilicalis), composed primarily of mucopolysaccharides such as hyaluronic acid and chondroitin sulfate. This matrix encases the umbilical vessels and the urachus.[7]

Three vessels develop within the umbilical cord: 2 umbilical arteries and 1 umbilical vein. Both right and left umbilical veins are present initially, but the right umbilical vein undergoes regression by the 6th week of intrauterine life.[8]

The urachus, a fibrous remnant of the allantois, traverses the umbilical cord.[9] Unique to fetal life, this structure connects the urinary bladder to the allantoic cavity and occupies the space of Retzius, positioned between the peritoneum posteriorly and the transverse fascia anteriorly. The urachus functions as a fetal drainage conduit for the urinary bladder (see Image. Umbilical Cord).[10]

Function

The umbilical cord operates as a bidirectional conduit, transporting oxygen, nutrients, and oxygenated blood from the placenta to the fetus, while returning deoxygenated blood from the fetus to the placenta.[11] Umbilical arteries convey deoxygenated blood from fetal circulation toward the placenta. Approximately 5 mm proximal to the placental insertion, the 2 arteries converge to form a vascular anastomosis known as the Hyrtl anastomosis.[12] The primary role of this vascular network is to equalize blood flow and pressure between the umbilical and placental arteries.[13] Upon placental entry, the umbilical arteries divide into smaller branches termed "chorionic vessels."

Embryology

In the 3rd week of embryonic development, gastrulation initiates differentiation of the germinal tissues into 3 distinct layers: outer ectoderm, intraembryonic mesoderm, and inner endoderm.[14][15] Formation of the umbilical cord unfolds across 3 developmental stages and coincides with the gastrulation process (see Image. Embryo at Approximately 6 Weeks).

Stage I: Formation of the Primitive Umbilical Ring

The embryonic surface begins to protrude into the amniotic cavity concurrent with the folding of the embryonic disc. As folding progresses, the amniotic-ectodermal junction—a firm connection between the amnion and ectoderm—shifts to occupy the ventral aspect of the embryo. The line of reflection between the amnion and ectoderm adopts an oval contour, designated as the primitive umbilical ring.

Stage II: Development of the Primitive Umbilical Cord

By the 5th week of pregnancy, constriction of the primitive umbilical ring gives rise to a tubular sheath. This sheath, termed the "primitive umbilical cord," encloses the body stalk, the yolk sac and its associated vessels, along with a segment of the allantois.

Stage III: Emergence of the Definitive Umbilical Cord

Elongation of the umbilical cord characterizes this stage, accompanied by primary modifications of its internal structures. Differentiation of the extraembryonic mesoderm within the body stalk gives rise to the Wharton jelly, a mucoid connective tissue that gradually develops to form the main bulk of the umbilical cord. Degeneration of the residual extraembryonic coelom within the cord progresses simultaneously.

Obliteration of the yolk sac occurs along with regression of the vitellointestinal duct, which initially connects the yolk sac to the midgut. The distal segment of the allantois also becomes obliterated. In contrast, the allantoic vessels remain and elongate to form the umbilical vessels.

A portion of the midgut loop enters the umbilical cord by the 6th week of development, producing a physiological herniation. Return of this herniated midgut to the abdominal cavity typically occurs after the 10th week of pregnancy.[16]

Blood Supply and Lymphatics

The umbilical cord, in coordination with the placenta, supports and regulates fetal circulation. The 2 umbilical arteries originate from the internal iliac arteries of the fetus and pass into the umbilical cord before branching further within the placenta. Each artery divides into smaller arterioles at the placental level, giving rise to chorionic vessels that distribute blood to the chorionic villi, the fetal component of the placenta. Capillary networks within the villi coalesce into venules that merge to form the umbilical vein.

The umbilical vein transports oxygenated blood and nutrients from the maternal circulation to the fetus.[17] Entry of the vein into the fetal abdomen directs blood toward both the liver parenchyma and the ductus venosus. From the ductus venosus, blood flows into the inferior vena cava and then to the fetal heart. A substantial portion of this oxygen-rich blood is shunted through the foramen ovale into the left atrium, bypassing the pulmonary circulation.[18]

Both placental intervillous circulation and umbilical circulation undergo gradual development as fetal growth progresses, reaching functional maturity by the end of the 1st trimester. Umbilical vein blood flow increases steadily from approximately 63 mL/min at 20 weeks of gestation to more than 370 mL/min by 38 weeks.[19] When normalized to estimated fetal weight, an inverse relationship emerges between umbilical vein blood flow and estimated fetal weight. At midgestation, umbilical blood accounts for approximately 30% of total fetal cardiac output. During the 3rd trimester, the proportion of umbilical blood flow relative to fetal weight declines markedly, falling to 20% or less.

Discussion of lymphatic drainage within the placenta and umbilical cord in scientific literature is currently limited. Recent findings have identified D2-40 expression within the placental stroma as a key feature of fetal lymphatic drainage. This marker is associated with podoplanin-expressing cells involved in the formation of a reticular network resembling lymphatic architecture. These specialized cells appear to mediate lymphatic drainage from both the umbilical cord and the placenta.[20]

Nerves

Absence of innervation characterizes the umbilical cord throughout all stages of embryonic development. In contrast, the umbilical vessels are surrounded by a dense neural plexus, which has been proposed to influence their vasomotor responses.[21] Regulation of smooth muscle tone within the umbilical vasculature depends on both locally secreted vasoactive substances within the vessel wall and agents delivered via the fetal circulation. Nitric oxide and prostacyclin are essential for maintaining low vascular resistance in the umbilical and placental circulations. In contrast, catecholamines serve as the principal mediators of umbilical vessel vasoconstriction following parturition.[22]

Muscles

The bulk of the umbilical cord is composed of the Wharton jelly, reflecting the absence of voluntary skeletal muscle within the structure. However, the umbilical vasculature contains multiple smooth muscle layers with varying composition and thickness. The walls of the umbilical vessels are organized into 3 primary layers: tunica externa, tunica media, and tunica interna.

Tunica Externa

Also known as the tunica adventitia, this outermost layer consists of fibrous and elastic connective tissue with variable amounts of collagen and elastic fibers. Dense connective tissue predominates near the tunica media, while a transition to looser connective tissue occurs toward the periphery of the vessel wall. Greater connective tissue density characterizes the tunica externa of the umbilical arteries compared to that of the umbilical vein.[23]

Tunica Media

The tunica media occupies the intermediate position within the wall of the umbilical vessels. Composed primarily of smooth muscle, this layer forms the muscular bulk of the vessel and provides mechanical support. Active regulation of vessel diameter occurs within the tunica media, influencing both blood flow and pressure through the umbilical cord. The tunica media is typically the thickest among the vascular layers. The tunica media is significantly thicker in the umbilical arteries than in the umbilical vein. Internal and external elastic membranes are well defined in the umbilical arteries but are often less distinct or absent in the walls of the umbilical vein.[24]

Tunica Interna

Also referred to as the tunica intima, the tunica interna constitutes the innermost layer of the umbilical vasculature. This layer consists of simple squamous epithelium resting on a basement membrane composed of connective tissue rich in elastic fibers. Together, these elements form the endothelium of the umbilical vessels. The tunica interna of the umbilical vein contains valves that promote unidirectional blood flow and prevent regurgitation. These valves are absent in the walls of the umbilical arteries.[25] Human umbilical artery smooth muscle cells are routinely isolated postdelivery and employed as experimental models to advance the understanding of smooth muscle cell physiology.[26]

Physiologic Variants

Umbilical Cord Coiling Patterns

One of the most frequent morphological variations of the umbilical cord involves differences in its helical coiling pattern. The 4 principal types include rope (the most common), undulating, segmented, and linked configurations. The degree of coiling is quantified by the umbilical cord index, typically measuring around 0.2 coils/cm.

Hypercoiling is defined by an umbilical cord index exceeding 0.3 coils/cm and occurs in approximately 6% to 21% of all pregnancies.[27] Abnormal coiling patterns are strongly associated with fetal vascular obstruction, a condition that may lead to complications such as fetal thrombi, avascular villi, and villous stromal vascular karyorrhexis.[28]

False Knots of the Umbilical Cord

False knots appear as bulging masses along the surface of the umbilical cord. These formations result from redundant loops or folds of the umbilical vessels within the cord, commonly referred to as "folds of Hoboken."[29] Excessive torsion of the cord may exaggerate these bulges on prenatal ultrasonography, mimicking the appearance of true knots. Uneven distribution of the Wharton jelly contributes to the formation of these structures. Thick accumulations create the bulges, while thinner regions produce intervening constrictions. This physiologic variation does not impair fetal positioning, umbilical blood flow, or vascular pressure. Consequently, false knots do not pose a significant risk to the fetus.[30]

Single Umbilical Artery

A single umbilical artery (SUA) occurs infrequently but is more commonly observed in multiparous patients than in nulliparous ones. Presence of an SUA alters fetal hemodynamics, prompting close monitoring of vascular parameters such as the resistance index, pulsatility index, and systole-to-diastole ratio.[31] Absence of the left umbilical artery is more frequent than absence of the right.[32] Identification of an SUA on prenatal ultrasound is associated with an increased incidence of congenital anomalies, particularly affecting the cardiac, renal, gastrointestinal, and skeletal systems when the left artery is absent.[32][33][34][35][36] Additional studies have suggested a possible association between SUA and an elevated risk of urinary tract infections in neonates.[37]

Surgical Considerations

Anesthetic Considerations

Uterine myometrial tone and maternal blood pressure exert direct influence on uterine artery blood flow, which subsequently affects placental and umbilical circulation, thereby altering fetal oxygen delivery. Volatile anesthetics typically reduce myometrial tone and lower maternal blood pressure, resulting in diminished placental perfusion and decreased fetal oxygenation. Severe maternal hypercapnia induces vasoconstriction, which may impair venous return through the umbilical vein and contribute to fetal hypoxia.

Obstetric Considerations

Ostetric ultrasonography is employed during pregnancy to screen for fetal growth restriction. Growth-restricted fetuses face an increased risk of uteroplacental insufficiency, a condition associated with adverse obstetric outcomes, including stillbirth. Umbilical artery Doppler ultrasonography serves as a tool to evaluate fetoplacental vascular resistance. Elevated resistance indicates the presence and severity of placental insufficiency. Spectral Doppler waveforms of the umbilical arteries are used to calculate the pulsatility index, which quantifies downstream resistance, and the resistance index, which measures the proportion of systolic flow maintained during diastole. Elevated pulsatility and resistance index values indicate increased placental vascular resistance and impaired fetoplacental perfusion.

Progressive uteroplacental insufficiency follows a predictable pattern of hemodynamic decline. Initial findings include pulsatility index values exceeding the 95th percentile. Subsequent reduction in end-diastolic flow increases the systolic-to-diastolic flow ratio. Complete absence of end-diastolic flow may then occur, followed by reversed end-diastolic flow, indicating severe placental compromise.[38] Optimal Doppler measurements should be obtained from a free loop of the umbilical cord using angle-independent indices. These umbilical artery parameters serve as reliable indicators of fetoplacental circulatory integrity and inform critical obstetric management decisions.

Umbilical Vascular Access in Neonatal Resuscitation

The umbilical vein serves as the primary access point for vascular cannulation in the neonate. This blood vessel remains patent for approximately a week following delivery and provides a reliable route for administering intravenous fluids and medications in newborns requiring advanced resuscitative support. During umbilical vein catheterization, the catheter is advanced through the ductus venosus and inferior vena cava, with the tip positioned near the right atrium of the heart.[39]

Umbilical artery lines may also be placed during the 1st week of life for resuscitative and monitoring purposes. Umbilical artery catheterization provides direct access for continuous blood pressure monitoring, serial arterial blood gas sampling, and contrast-enhanced angiographic studies.[40]

Clinical Significance

Various umbilical cord abnormalities may pose serious threats to fetal health or result in fetal demise. Therefore, early detection, accurate diagnosis, and appropriate management are of critical clinical importance. The section below discusses some of these abnormalities.

Velamentous Insertion

Velamentous cord insertion (VCI) occurs when the umbilical cord inserts at the periphery of the placenta rather than centrally. The term also refers to cases in which the cord vessels separate and travel between the amnion and chorion, without the protection of the Wharton jelly, before reaching the placental disc (see Image. Velamentous Cord Insertion).[41]

VCI occurs in singleton gestations conceived naturally but is more prevalent in multiple gestations and in pregnancies achieved through in vitro fertilization (IVF).[42] Although the precise mechanism remains unclear, the leading hypothesis proposes that asymmetric placental development, marked by proliferation of one placental pole and involution of the other, results in peripheral displacement of the umbilical cord insertion site.

VCI is associated with impaired placental perfusion and fetal oxygen delivery because the unprotected vessels are vulnerable to external compression and rupture. This condition increases the risk of numerous obstetric complications, including the delivery of small-for-gestational-age neonates [relative risk (RR) 1.93], preeclampsia (RR 1.85), placental abruption (RR 2.94), preterm delivery (RR 2.14), emergency cesarean section (RR 2.03), low Apgar scores, and stillbirth (RR 4.12).[43] Additionally, VCI heightens the risk of complications during the 3rd stage of labor, with increased rates of manual placental removal (5-fold), curettage (3-fold), and postpartum hemorrhage (2-fold).[44]

Of particular concern is the strong association between VCI and vasa previa, a condition marked by exposed fetal vessels crossing the internal cervical os. These vessels are highly susceptible to tearing during membrane rupture or active labor, with potentially catastrophic fetal hemorrhage.

Four-Vessel Umbilical Cord

The normal umbilical cord contains 3 vessels: 2 umbilical arteries and 1 umbilical vein. The right umbilical vein typically regresses by the 7th week of gestation, leaving only the left umbilical vein patent. Rarely, the umbilical cord retains all 4 vessels—2 arteries and 2 veins. This anomaly is associated with a higher incidence of congenital abnormalities, particularly affecting the cardiovascular and gastrointestinal systems.[45] Persistence of both umbilical veins results in a condition known as persistent right umbilical vein. This abnormality is believed to arise from disruptions during early embryogenesis, possibly related to folate deficiency in the 1st trimester. The condition may have teratogenic effects and has been linked to adverse fetal outcomes.[46]

True Knots of the Umbilical Cord

True knots are genuine entanglements of the umbilical cord that form during gestation, often in the early stages of pregnancy. Several risk factors contribute to the development of these knots, including polyhydramnios, long umbilical cord length, and increased fetal mobility. These factors increase torsional stress on the cord and predispose to supercoiling and looping. True knots may compromise blood flow through the umbilical vessels, particularly during labor, and are associated with an increased risk of fetal hypoxia and, in severe cases, fetal demise.[47]

Abnormal Umbilical Cord Length

An umbilical cord is considered abnormally short when its length measures less than 40 cm. A markedly short cord may result in premature placental separation, which can interrupt fetal circulation and lead to intrauterine hemorrhage and fetal death.[48]

By contrast, an umbilical cord longer than 65 to 70 cm is considered excessively long. A long umbilical cord may encircle the fetal neck, increasing the risk of adverse outcomes. A long umbilical cord may also descend into the cervix during malpresentation, leading to umbilical cord prolapse.[49]

Omphalocele

Also known as exomphalos, an omphalocele is a congenital abdominal wall defect in which bowel and, at times, other abdominal organs herniate into a membranous sac at the base of the umbilical cord. This anomaly results from the failure of the physiological umbilical herniation to resolve during development.[50] Surgical correction is typically undertaken in the neonatal period to prevent complications, such as intestinal obstruction.

Umbilical Cysts

Umbilical cord cysts are classified as either true cysts or pseudocysts. These lesions are typically identified near the fetal insertion site of the umbilical cord and are most commonly detected during the 1st trimester. Most resolve spontaneously by the end of the 12th week of gestation. Umbilical cysts occur in approximately 3.4% of all pregnancies.

The most frequently encountered type is the pseudocyst, also referred to as a "Wharton jelly cyst." These formations lack an epithelial lining and arise from focal edema or mucoid degeneration within the Wharton jelly. Pseudocysts may appear as single or multiple lesions, typically measuring less than 2 cm in diameter.

In contrast, true cysts of the umbilical cord are derived from persistent embryologic structures, most commonly the omphalomesenteric duct or the allantois. True cysts possess a distinct epithelial lining, which differentiates them histologically from pseudocysts.[51]

Although many umbilical cysts are transient and benign, their presence, particularly when persistent into the 2nd or 3rd trimester, may indicate underlying chromosomal abnormalities. Reported associations include trisomy 13, trisomy 18, imperforate anus, and angiomyxoma of the umbilical cord.[52] Fetal karyotyping and amniocentesis may be warranted for further evaluation in such cases.

Delayed Umbilical Cord Separation

Normal umbilical cord separation occurs within the first few weeks of life, although the timing varies. Separation is considered delayed when it occurs more than 3 weeks after delivery. Multiple factors contribute to delayed detachment, including the topical application of antiseptics such as alcohol, antibiotics, or triple dye. Pathological causes include localized or systemic infections, immune disorders such as leukocyte adhesion deficiency, and congenital anomalies like urachal remnants. Studies also associate delayed separation with prematurity, cesarean delivery, and low birth weight. Further evaluation is warranted in neonates with delayed cord separation accompanied by skin infections or persistent umbilical anomalies, including urachal remnants.

Umbilical Granuloma

An umbilical granuloma is a benign vascular lesion that may develop after the umbilical cord detaches. These nodules, typically measuring around 5 mm in diameter, form as a result of excessive fibroblast proliferation and granulation tissue at the base of the umbilicus.[53] Grossly, these lesions have a moist, red, strawberry-like appearance due to prominent capillaries on their surface.[54] The standard treatment is chemical cauterization using silver nitrate, applied carefully to avoid injury to the surrounding skin.