Anatomic depiction of the umbilical cord is possible by ultrasound and is part of most anatomic surveys in the second and third trimesters. Abnormal umbilical cord morphology may be a variant of normal or may be associated with fetal karyotypic abnormalities, genetic syndromes, structural fetal anomalies, or untoward pregnancy complications such as fetal growth delay. Additionally, Doppler flow studies of the umbilical artery for assessment of vascular impedance in complicated pregnancies are an important diagnostic tool.
Formation of the Umbilical Cord
The primitive umbilical ring represents an oval line of reflection between the amnion and embryonic ectoderm.68 During the fifth developmental week, the connecting stalk (containing the umbilical vessels and allantois) and the yolk stalk (containing the vitelline vessels and the original attachment of the yolk sac) pass through the ring.68 The yolk sac is then located between the amnion and chorion. With rapid growth of the amniotic cavity, the amnion encases the connecting stalk and yolk stalk, which coalescence into the umbilical cord.68 The yolk sac, located in the chorionic cavity, is connected to the umbilical cord by its stalk. As the amnion grows to meet the chorion, the yolk sac gradually recedes.68 The embryonic allantoic arteries form the umbilical arteries. The left allantoic vein develops into the umbilical vein, whereas the right allantoic vein regresses.44 The omphalomesenteric vessels are obliterated. The result is an umbilical cord, covered by amnion and containing a single umbilical vein, and 2 umbilical arteries supported in Wharton jelly, a mesodermally derived mucoid matrix.44 The cord itself is devoid of lymphatic vessels and nerves.44 Although the walls of the umbilical vessels have a large proportion of muscle, they lack collagen and elastin and so are able to change configuration with changes in osmotic pressure in the amniotic fluid.
Structure and Function of the Umbilical Cord
The umbilical cord has a variable length, and normally contains 2 umbilical arteries and a single large umbilical vein surrounded by a clear mucoid connective tissue, Wharton jelly. Wharton jelly surrounding the vessels apparently has the function of protecting the vessels from undue torsion and compression.69 A layer of amnion covers the umbilical cord except near the fetal insertion, where an epithelial covering is substituted. The arteries wind around the umbilical vein in a spiral fashion because the umbilical vessels are longer than the cord.6 There may be a number of foldings, loops, and tortuosities producing protrusions or false knots on the cord surface. Both the helical pattern (coiling) of the cord vessels and the presence of Wharton jelly confer turgor to the cord, which helps prevent kinking and cord prolapse. In approximately 1% of pregnancies, true knots are found, which may be an incidental finding or can be associated with untoward pregnancy outcomes.6
Oxygenated blood flows in the umbilical vein from the placenta to the fetus and, on reaching the fetal abdominal wall, passes through the liver posteriorly and cephalad to terminate at the portal sinus (the main left portal vein). Deoxygenated blood from the fetal aorta passes to the hypogastric arteries, which wind superiorly and medially along the superolateral margin of the bladder, to enter the cord as the umbilical arteries, which carry blood back to the placenta.
SONOGRAPHIC ANATOMY OF THE NORMAL CORD
The umbilical stalk and the yolk sac are seen as early as 7 weeks adjacent to the anterior abdominal wall of the developing fetus. The yolk sac is extra-amniotic (Figure 7-41). In the late first trimester and in the second and third trimesters, the umbilical cord is readily visualized. As imaged in long axis, the cord may be seen as a series of parallel lines and shorter angled, linear interfaces arising from the umbilical arteries that wrap around the central vein (Figure 7-42A and B). In a cross-sectional view, the arteries and umbilical vein may be seen as 3 separate circular lucencies (Figure 7-43). Pulsations of the cord, occurring at the same rate as fetal heart rate, may be seen in real time.
Yolk sac. The yolk sac (YS) is extra-amniotic. (Arrow, amnion; F, fetus.)
A: Umbilical cord in a longitudinal view in a 20-week pregnancy with 2 arteries (arrows) and 1 vein (v). B: Color-flow Doppler demonstrates flow in different directions from the 2 arteries (red) and vein (blue).
Sonographic image of normal umbilical cord in cross section containing 2 arteries (arrows) and 1 vein (v).
Because there are different directions of flow in the umbilical cord, a color Doppler image will demonstrate one color in the vein and another in the arteries (see Figure 7-42B). The umbilical arteries course laterally around the fetal urinary bladder and may be identified with either color or power Doppler (Figure 7-44). The cord insertion into the placenta may appear as a V- or U-shaped sonolucent area arising from the chorionic plate.
Normal umbilical cord with the umbilical arteries coursing laterally around the urinary bladder as depicted by color-flow Doppler.
Eccentric or marginal cord insertions, velamentous insertions, and vasa previa were mentioned earlier in this chapter. At the insertion of the cord into the anterior abdominal wall of the fetus (fetal insertion), the origins of the umbilical vein and hypogastric arteries may be seen. Weakness in the anterior abdominal wall of the involuted right umbilical vein is one etiology for gastroschisis, a full-thickness abdominal wall defect with an eccentric herniation of bowel.
Persistent right umbilical vein is identified sonographically by (1) the umbilical vein being connected to the right portal vein rather than the left; (2) the position of the intrahepatic portion of the umbilical vein being lateral to the gall bladder, rather than medial; and (3) the portal vein curving toward the stomach rather than toward the liver.70 The incidence of persistent right umbilical vein is approximately 0.2% and is typically an isolated finding. If no additional structural anomalies are detected, this condition represents a benign variant with a favorable prognosis.71,72
Abnormalities of Cord Length
The average normal umbilical cord length is about 55 cm but ranges from 30 to 90 cm in length and is 1 to 2 cm in diameter.6,44,46 Extremes of cord length may occur from apparently no cord (achordia) to lengths of up to 300 cm.73 Achordia has been associated with abdominal wall/ limb-body wall defects.46 Excessively long cords may predispose to vascular occlusion by thrombi, true knots, entanglement, and also to cord prolapse after membrane rupture. Excessively short umbilical cords have a length of 32 cm or less.44 Short cords are associated with congenital anomalies, intrapartum fetal distress, and fetal death.74 Uterine inversion and placental abruption may occur as a result of a short umbilical cord. Childhood mental and motor impairment, and depressed intelligence quotient values are correlated with excessively short cords.44,46 It is uncertain whether the length of the umbilical cord is the cause or the result of an underlying central nervous system problem.44,46
Sonographic delineation of the umbilical cord with color or power Doppler sonography may be helpful in a qualification assessment of cord length. Thus, a subjective assessment of cord length is possible (see Figure 7-42).
Abnormalities of Cord Position
Loops of umbilical cord usually lie anterior to the fetal abdominal wall and adjacent to the limbs. In a number of instances, however, loops of cord may encircle the fetal neck or limbs, or loops of cord may lie between the fetal presenting part and the lower uterine segment (funic presentation).
A single nuchal cord is common in pregnancy and is reported in 25% to 30% of pregnancies (Figure 7-45). Nuchal cords are generally not associated with significant fetal compromise or complications.75 However, there have been pregnancy complications reported in association with nuchal cords such as a nonreassuring fetal heart rate tracing, meconium staining, 5-minute Apgar score less than 7, assisted ventilation, and cerebral palsy.75 Multiple loops are seen in only 2% of pregnancies.
Nuchal cord depicted by color-flow Doppler in a 21-week pregnancy.
Occasionally, loops of the cord may be seen lying between the fetal presenting part and the lower segment, referred to as cord or funic presentation. It is important to recognize this because such a position predisposes to cord prolapse and possible fetal death at the time of rupture of the membranes. Funic presentation is more common with malpresentations, such as breech or transverse lie. The condition, especially before 32 weeks, may often be transient and clinically insignificant; however, one should look for causes that may produce a persistent cord presentation and risk cord prolapse. These causes include a marginal cord insertion from the caudal margin of low-lying placenta, uterine structural anomalies such as fibroids or uterine adhesions, or congenital malformations that may prevent the fetus from engaging well into the lower uterine segment.
Although the normal umbilical cord contains 2 umbilical arteries, a single umbilical artery (SUA) may be seen in approximately 1% of all newborn singletons and occurs more frequently in twins and abortuses (Figure 7-46A,B,C, and D).46,76 Heifetz in an extensive review found the prevalence of SUA to be 0.63%.77 SUA occurs more frequently in women with diabetes, epilepsy, preeclampsia, antepartum hemorrhage, hydramnios, and decreased amniotic fluid.78 Fetal malformations are often found with SUA with an incidence that varies in the literature at a range between 25% and 50%, and with an increased frequency of renal and cardiac anomalies, fetal growth restriction, and karyotypic abnormalities.44,79,80
A: Sonographic image of 2-vessel umbilical cord in cross section containing a single artery (a) and vein (v) at 32 weeks' gestation. B: Single umbilical artery at 27 weeks. C: Umbilical cord in a longitudinal view with a single artery (a) and vein (v). D: A single umbilical artery coursing laterally around the urinary bladder depicted by power Doppler.
A single umbilical artery may be the consequence of aplasia or of atrophy of one artery.46 Routine ultrasound screening is a reliable tool to identify umbilical vessels either as a free loop or with color or power Doppler at the level of the fetal urinary bladder. Caution should be exercised when evaluating the cord vessels close to the chorionic plate as the 2 arteries may fuse into a single trunk near the cord insertion into the placenta.44,46
To summarize the discussion of SUA, the following points are made:
There is a positive association of fetal anomalies with SUA involving any organ system.
If SUA is associated with a detected fetal anomaly, there is a high risk of aneuploidy.
A fetus with SUA and no detectible anomaly is still at risk (although small) for a clinical abnormality, and clinical judgment needs to guide decisions as to whether or not to pursue invasive prenatal diagnosis for karyotyping.
Even if isolated, pregnancies with a single umbilical artery are at increased risk for fetal growth delay, and serial sonography may be considered.
True cord cysts may be due to allantoic or omphalomesenteric duct remnants, whereas false cysts result from focal liquefaction of Wharton jelly, often occurring within a regional zone of thickening of the jelly. All are very uncommon in the second and third trimesters. Cysts can be seen more frequently, although they are still uncommon, in the first trimester. Most spontaneously resolve by the end of the first trimester. Such cysts, even when large, usually do not jeopardize fetal circulation, but isolated case reports of cord accidents from cysts and other focal cord lesions indicate the need for ongoing monitoring for fetuses that have any cord lesion. In pregnancies where multiple cysts are identified or cysts identified in the first trimester persist are at increased risk for structural and karyotypic abnormalities.44 Sachs et al81 reported the prenatal diagnosis of a 5-cm cystic mass within the umbilical cord several centimeters from the abdominal wall at 21 weeks of gestation. Visualization of vessels in the lateral wall of the mass and an intact anterior abdominal wall allowed exclusion of other pathologies.
True knots may be seen in pregnancies with excessively long umbilical cords and in pregnancies complicated by hydramnios (Figure 7-47). They are formed by the fetus moving through a portion of cord. The incidence of true knots varies in the literature with an incidence ranging from 0.04% to 1.5%.44 True knots are identified more frequently in monoamniotic twin gestations due to the lack of a dividing membrane. False knots of the umbilical cord represent vascular redundancies of the umbilical vessels. False knots are of little clinical significance.46
True knot in the umbilical cord diagnosed at cesarean delivery.
Hematomas of the umbilical cord may be the result of amniocentesis, fetal blood sampling, or rupture of a varix.44 Cord hematomas are occasionally seen with short cords, entangling, and trauma.46 In many instances of cord hematoma, no etiology is elucidated. Thrombosis of umbilical vessels may accompany cord compression, torsion, stricture, or hematoma formation. These usually occur in the umbilical vein.44 Other neoplasms of the umbilical cord, which are quite rare, include angiomyxomas, myxosarcomas, and choriomyxomas, and in fact probably represent hemangiomas. Hemangiomas are rare and described as rounded or ovoid swellings in the cord.44 Sonographically, hemangiomas may appear as echogenic masses related to the cord.82 They may be associated with elevated α-fetoprotein values. Teratomas represent primary tumors of the cord with cystic and solid components.44
Umbilical hernia is one of the most commonly encountered abnormalities in early infancy.83 Umbilical hernia has been reported as being more common in trisomy 21, congenital hypothyroidism, mucopolysaccharidoses, and the Beckwith syndrome.83 Sonographically, umbilical hernia may be recognized as a protrusion from the anterior abdominal wall, with a normal insertion of the umbilical vessels.
Omphalocele and gastroschisis represent abnormalities of closure of the anterior abdominal wall. With omphalocele there is a midline umbilical defect with protrusion of abdominal structures, such as bowel and liver, into the base of the umbilical cord. This produces a sonographic appearance of a mass adjacent to the anterior abdominal wall, covered with a membrane, and into the apex of which the umbilical cord appears to insert. There is a high incidence of other anomalies (eg, intestinal, cardiac, and renal anomalies). Recent studies have shown a difference in the risk of aneuploidy in fetuses with small omphaloceles that tend to contain bowel only, whereas large ones contain both bowel and liver.
With gastroschisis, a right paraumbilical abdominal wall defect results in protrusion of bowel and other intraabdominal contents into the amniotic fluid. The cord normally inserts just to the left of the defect; therefore, gastroschisis will appear as a complex mass adjacent to the base of the cord. The exteriorized bowel loops are not covered by a membrane, and therefore are directly exposed to amniotic fluid. A more detailed discussion of the entities can be found in Chapter 17 on sonography of fetal gastrointestinal tract anomalies.
Recent studies have suggested that fetuses with a noncoiled umbilical cord are at greater risk for karyotypic abnormalities and poor fetal conditions when compared with controls.84 The detection of a coiled cord can be enhanced with the use of color Doppler sonography to show the "pitch" (number of coils per unit length) of umbilical arteries as they course around the umbilical vein. Further studies devised a coiling index of the cord by dividing the number of complete coils by the length of the cord. Fetuses with low coiling indices had poorer outcomes.85 Overcoiling or undercoiling of the cord is reported in association with poorer perinatal outcomes.44