Most umbilical cords at delivery are 40 to 70 cm long, and very few measure <30 cm or >100 cm. Cord length is influenced positively by both amnionic fluid volume and fetal mobility (Miller, 1982). In retrospective studies, short cords have been linked with congenital malformations and intrapartum distress (Baergen, 2001; Krakowiak, 2004; Yamamoto, 2016). Excessively long cords are linked with cord entanglement or prolapse and with fetal anomalies (Olaya-C, 2015; Rayburn, 1981).
Because antenatal determination of cord length is technically limited, cord diameter has been evaluated as a predictive marker for fetal outcomes. Some have linked lean cords with poor fetal growth and large-diameter cords with macrosomia (Proctor, 2013). However, the clinical utility of this parameter is still unclear (Barbieri, 2008; Cromi, 2007; Raio, 1999b, 2003).
Cord coiling characteristics have been reported but are not currently part of standard sonographic evaluation. Usually the umbilical vessels spiral through the cord in a sinistral, that is, left-twisting direction (Fletcher, 1993; Lacro, 1987). The number of complete coils per centimeter of cord length is termed the umbilical coiling index—UCI (Strong, 1994). A normal, antepartum, sonographically derived UCI is 0.4, and this contrasts with a normal, postpartum, physically measured value of 0.2 (Sebire, 2007). UCIs <10th percentile are considered hypocoiled, and those >90th percentile are hypercoiled. Clinically, the significance of coiling extremes is controversial. Some studies evaluating large, unselected cohorts find no associations between UCI values and poor neonatal outcome (Jessop, 2014; Pathak, 2010). In others, extremes are linked with various adverse outcomes but most consistently with intrapartum fetal heart rate abnormalities, preterm labor, or fetal-growth restriction (Chitra, 2012; de Laat, 2006; Predanic, 2005; Rana, 1995).
Counting cord vessel number is a standard component of anatomical evaluation during fetal sonographic examination and immediately after delivery (Fig. 6-5). Embryos initially have two umbilical veins. In the first trimester, the right vein typically atrophies to leave one large vein to accompany the two, thick-walled umbilical arteries. Four-vessel cords are rare and often associated with congenital anomalies (Puvabanditsin, 2011). If it is an isolated finding, however, prognosis can be good (Avnet, 2011).
Two umbilical arteries are typically documented sonographically in the second trimester. They encircle the fetal bladder (asterisk) as extensions of the superior vesical arteries. In this color Doppler sonographic image, a single umbilical artery, shown in red, runs along the bladder wall before joining the umbilical vein (blue) in the cord. Below this, the two vessels of the cord, seen as a larger red and smaller blue circle, are also seen floating in a cross section of a cord segment.
The most common aberration is that of a single umbilical artery (SUA), with a cited incidence of 0.63 percent in liveborn neonates, 1.92 percent with perinatal deaths, and 3 percent in twins (Heifetz, 1984a). Fetuses with major malformations frequently have a single artery. Thus, its identification often prompts consideration for targeted sonography and possibly fetal echocardiography. The most frequent anomalies are cardiovascular and genitourinary (Hua, 2010; Murphy-Kaulbeck, 2010). In an anomalous fetus, a single artery greatly increases the aneuploidy risk, and amniocentesis is recommended (Dagklis, 2010; Lubusky, 2007).
If target sonography finds otherwise normal anatomy, an isolated single artery in an otherwise low-risk pregnancy does not significantly increase the fetal aneuploidy risk. However, as in isolated finding, it has been associated with fetal-growth restriction and perinatal death in some but not all studies (Chetty-John, 2010; Gutvirtz, 2016; Hua, 2010; Murphy-Kaulbeck, 2010; Voskamp, 2013). Thus, while clinical monitoring of growth is reasonable, the value of sonographic surveillance is unclear.
A rare anomaly is that of a fused umbilical artery with a shared lumen. It arises from failure of the two arteries to split during embryological development. The common lumen may extend through the entire cord, but, if partial, it is typically found near the placental insertion site (Yamada, 2005). In one report, these were associated with a higher incidence of marginal or velamentous cord insertion, but not congenital fetal anomalies (Fujikura, 2003).
Found in most placentas, the Hyrtl anastomosis is a connection between the two umbilical arteries and lies near the cord insertion into the placenta. This anastomosis acts as a pressure-equalizing system between the arteries (Gordon, 2007). As a result, redistribution of pressure gradients and blood flow improves placental perfusion, especially during uterine contractions or during compression of one umbilical artery. Fetuses with a single umbilical artery lack this safety valve (Raio, 1999a, 2001).
Several structures are housed in the umbilical cord during fetal development, and their remnants may be seen when the mature cord is viewed transversely. Indeed, Jauniaux and colleagues (1989) sectioned 1000 cords, and in one fourth of the specimens, they found remnants of vitelline duct, allantoic duct, and embryonic vessels. These were not associated with congenital malformations or perinatal complications.
Cysts occasionally are found along the course of the cord. They are designated according to their origin. True cysts are epithelium-lined remnants of the allantoic or vitelline ducts and tend to be located closer to the fetal insertion site. In contrast, the more common pseudocysts form from local degeneration of Wharton jelly and occur anywhere along the cord. Both have a similar sonographic appearance. Single umbilical cord cysts identified in the first trimester tend to resolve completely, however, multiple cysts may portend miscarriage or aneuploidy (Ghezzi, 2003; Hannaford, 2013). Cysts persisting beyond this time are associated with a risk for structural defects and chromosomal anomalies (Bonilla, 2010; Zangen, 2010).
The cord normally inserts centrally into the placental disc, but eccentric, marginal, or velamentous insertions are variants. Of these, eccentric insertions in general pose no identifiable fetal risk. Marginal insertion is a common variant—sometimes referred to as a battledore placenta—in which the cord anchors at the placental margin. In one population-based study, the rate was 6 percent in singleton gestations and 11 percent in twins (Ebbing, 2013). This common insertion variant rarely causes problems, but it and velamentous insertion occasionally result in the cord being pulled off during delivery of the placenta (Ebbing, 2015; Luo, 2013). In monochorionic twins, this insertion may be associated with weight discordance (Kent, 2011).
With velamentous insertion, the umbilical vessels characteristically travel within the membranes before reaching the placental margin (Fig. 6-6) The incidence of velamentous insertion approximates 1 percent but is 6 percent with twins (Ebbing, 2013). It is more commonly seen with placenta previa (Papinniemi, 2007; Räisänen, 2012). Antenatal diagnosis is possible sonographically, and with velamentous insertion, cord vessels are seen traveling along the uterine wall before entering the placental disc. Clinically, vessels are vulnerable to compression, which may lead to fetal hypoperfusion and acidemia. Higher associated rates of low Apgar scores, stillbirth, preterm delivery, and small for gestational age have been noted (Ebbing, 2017; Esakoff, 2015; Heinonen, 1996; Vahanian, 2015). Accordingly, monitoring of fetal growth is reasonable either clinically or sonographically (Vintzileos, 2015).
Velamentous cord insertion. A. The umbilical cord inserts into the membranes. From here, the cord vessels branch and are supported only by membrane until they reach the placental disc. B. When viewed sonographically and using color Doppler, the cord vessels appear to lie against the myometrium as they travel to insert marginally into the placental disc, which lies at the top of this image.
Last, with the very uncommon furcate insertion, umbilical vessels lose their protective Wharton jelly shortly before they insert. As a result, they are covered only by an amnion sheath and prone to compression, twisting, and thrombosis.
With this condition, vessels travel within the membranes and overlie the cervical os. There, they can be torn with cervical dilatation or membrane rupture, and laceration can lead to rapid fetal exsanguination. Over the cervix, vessels can also be compressed by a presenting fetal part. Fortunately, vasa previa is uncommon and has an incidence of 2 to 6 per 10,000 pregnancies (Ruiter, 2016; Sullivan, 2017). Vasa previa is classified as type 1, in which vessels are part of a velamentous cord insertion, and type 2, in which involved vessels span between portions of a bilobate or a succenturiate placenta (Catanzarite, 2001). Two other risks are conception with in vitro fertilization and second-trimester placenta previa, with or without later migration (Baulies, 2007; Schachter, 2003).
Compared with intrapartum diagnosis, antepartum diagnosis greatly improves the perinatal survival rate, which ranges from 97 to 100 percent (Oyelese, 2004; Rebarber, 2014; Swank, 2016). Thus, vasa previa is ideally identified early, although this is not always possible. Clinically, an examiner is occasionally able to palpate or directly see a tubular fetal vessel in the membranes overlying the presenting part. Effective screening for vasa previa begins during scheduled midtrimester sonographic examination. In suspicious cases, transvaginal sonography is added and shows cord vessels inserting into the membranes—rather than directly into the placenta—and vessels running above the cervical internal os (Fig. 6-7). Routine color Doppler interrogation of the placental cord insertion site, particularly in cases of placenta previa or low-lying placenta, may aid its detection. With this, the vessel waveform reflects the fetal heart rate. In one systematic review, the median prenatal detection rate was 93 percent (Ruiter, 2015).
Vasa previa. Using color Doppler, an umbilical vessel (red circle) is seen overlying the internal os. At the bottom, the Doppler waveform seen with this vasa previa has the typical appearance of an umbilical artery, with a pulse rate of 141 beats per minute.
Once vasa previa is identified, subsequent imaging is reasonable because 6 to 17 percent of cases ultimately resolve (Rebarber, 2015; Swank, 2016). Bed rest apparently has no added advantage. Antenatal corticosteroids can be provided as indicated or given prophylactically at 28 to 32 weeks’ gestation to cover possible urgent preterm delivery. Antenatal hospitalization may be considered at 30 to 34 weeks to permit surveillance and expedited delivery for labor, bleeding, or rupture of membranes. Data supporting this is limited, and admission may best serve women with risk factors that portend early delivery (Society for Maternal-Fetal Medicine, 2015). A few cases of antepartum fetoscopic surgery with vessel laser ablation are described (Hosseinzadeh, 2015; Johnston, 2014). However, current practice is early scheduled cesarean delivery. Robinson and Grobman (2011) performed a decision analysis and recommend elective cesarean delivery at 34 to 35 weeks’ gestation to balance the risks of perinatal exsanguination versus preterm birth morbidity. The Society for Maternal-Fetal Medicine (2015) considers planned cesarean delivery at 34 to 37 weeks’ gestation reasonable.
At delivery, the fetus is expeditiously delivered after the hysterotomy incision in case a vessel is lacerated during uterine entry. Delayed cord clamping is not encouraged.
In all pregnancies, otherwise unexplained vaginal bleeding either antepartum or intrapartum should prompt consideration of vasa previa and a lacerated fetal vessel. In many cases, bleeding is rapidly fatal, and infant salvage is not possible. With less hemorrhage, however, it may be possible to distinguish fetal versus maternal bleeding. Various tests may be used, and each relies on the increased resistance of fetal hemoglobin to denaturing by alkaline or acid reagents (Odunsi, 1996; Oyelese, 1999).
Knots, Strictures, and Loops
Various mechanical abnormalities in the cord can impede blood flow and sometimes cause fetal harm. Of these, true knots are found in approximately 1 percent of births. These form from fetal movement, and associated risks include hydramnios and diabetes (Hershkovitz, 2001; Räisänen, 2013). Knots are especially common and dangerous in monoamnionic twins, which are discussed in Chapter 45 (Unique Fetal Complications). When true knots are associated with singleton fetuses, the stillbirth risk is increased four- to tenfold (Airas, 2002; Sørnes, 2000).
Knots can be found incidentally during antepartum sonography, and a “hanging noose” sign is suggestive (Ramon y Cajal, 2006). Three-dimensional and color Doppler aid diagnostic accuracy (Hasbun, 2007). With these knots, optimal fetal surveillance is unclear but may include umbilical artery Doppler velocimetry, nonstress testing, or subjective fetal movement monitoring (Rodriguez, 2012; Scioscia, 2011). Allowing vaginal delivery is suitable, but abnormal intrapartum fetal heart rate tracings are more often encountered. That said, cesarean delivery rates are not increased, and cord blood acid-base values are usually normal (Airas, 2002; Maher, 1996).
In contrast, false knots form from focal redundancy and folding of an umbilical cord vessel. These lack clinical significance.
Cord strictures are focal narrowings of the diameter that usually develop near the fetal cord insertion site (Peng, 2006). Characteristic pathological features include an absence of Wharton jelly and stenosis or obliteration of cord vessels at the narrow segment (Sun, 1995). In most instances, the fetus is stillborn (French, 2005). Even less common is a cord stricture caused by an amnionic band.
Cord loops are frequently encountered and are caused by coiling around various fetal parts during movement. A cord around the neck—a nuchal cord—is common, and vaginal delivery is suitable. One loop is reported in 20 to 34 percent of deliveries; two loops in 2.5 to 5 percent; and three loops in 0.2 to 0.5 percent (Kan, 1957; Sørnes, 1995; Spellacy, 1966). During labor, up to 20 percent of fetuses with a nuchal cord have moderate to severe variable heart rate decelerations, and these are associated with a lower umbilical artery pH (Hankins, 1987). Cords wrapped around the body can have similar effects (Kobayashi, 2015). Despite their frequency, nuchal cords are not associated with greater rates of adverse perinatal outcome (Henry, 2013; Sheiner, 2006).
Last, a funic presentation describes when the umbilical cord is the presenting part in labor. These are uncommon and most often are associated with fetal malpresentation (Kinugasa, 2007). A funic presentation in some cases is identified with placental sonography and color flow Doppler (Ezra, 2003). Overt or occult cord prolapse can complicate labor. Thus, once identified at term, cesarean delivery is typically recommended.
Cord hematomas are rare and generally follow rupture of an umbilical vessel, usually the vein, and bleeding into the Wharton jelly. Hematomas have been associated with abnormal cord length, umbilical vessel aneurysm, trauma, entanglement, umbilical vessel venipuncture, and funisitis (Gualandri, 2008). Most are identified postpartum, but hematomas are recognized sonographically as hypoechoic masses that lack blood flow (Chou, 2003). Sequelae include stillbirth or intrapartum abnormal fetal heart rate pattern (Abraham, 2015; Barbati, 2009; Sepulveda, 2005; Towers, 2009).
Umbilical cord vessel thromboses are rare in utero events and seldom diagnosed antepartum. Approximately 70 percent are venous, 20 percent are venous and arterial, and 10 percent are arterial thromboses (Heifetz, 1988). These all have high associated rates of stillbirth, fetal-growth restriction, and intrapartum fetal distress (Minakami, 2001; Sato, 2006; Shilling, 2014). If these are identified antepartum as hypoechoic masses without blood flow, data from case reports support consideration of prompt delivery if of viable age (Kanenishi, 2013).
An umbilical vein varix can complicate either the intraamnionic or fetal intraabdominal portion of the umbilical vein. Sonographically and complemented by color Doppler, rare intraamnionic varices show cystic dilatation of the umbilical vein that is contiguous with a normal-caliber portion. Of complications, an intraamnionic varix may compress an adjacent umbilical artery or can rupture or thrombose. In cases without these, White and colleagues (1994) recommend fetal surveillance and delivery once fetal maturity is confirmed. However, data are limited and derived from case reports.
The rare umbilical artery aneurysm is caused by congenital thinning of the vessel wall with diminished support from Wharton jelly. Indeed, most form at or near the cord placental insertion site, where this support is absent. These are associated with single umbilical artery, trisomy 18, amnionic fluid volume extremes, fetal-growth restriction, and stillbirth (Hill, 2010; Vyas, 2016). At least theoretically, these aneurysms could cause fetal compromise and death by compression of the umbilical vein. These aneurysms may appear sonographically as a cyst with a hyperechoic rim. Within the aneurysm, color flow and spectral Doppler interrogation demonstrate either low-velocity or turbulent nonpulsatile flow (Olog, 2011; Sepulveda, 2003; Shen, 2007b). Although not codified, management may include fetal karyotyping, antenatal fetal surveillance, and early delivery to prevent stillbirth (Doehrman, 2014).