Introduction

The umbilical cord attached to the umbilicus, the only visible scar of our intimate contact with our mother prior to birth, helps provide the developing foetus with nutrients and oxygen as a crucial lifeline (Rana, Ebert, & Kappy, 1995). The umbilical cord (UC), along with the placenta, is the only organ that dies when life begins. Although the umbilical cord is one of the most interesting organs in humans, it remains one of the least studied structures (Khan & Thakur, 2019).

Umbilical Cord is vulnerable to compressions, traction, torsion, and kinking, which eventually affects the intrauterine life and foetal outcome. The cord contains 2 umbilical arteries and 1 umbilical vein with Wharton’s jelly as the connective tissue around the vessels. Wharton’s jelly contains a compound, “hyaluronan”, that has a crucial role in cord coiling and aids in the growth of the vessels in the UC along with its coiling. The role of acquired factors like the movement of the foetus in-utero and genetic factors in the normal coiling of the cord cannot be underrated. Umbilical Cord abnormalities (UCAs) often result from the mechanical compression leading to venous stasis, endothelial damage, and loss of perfusion to a larger area of placental gas exchanging villi, and ultimately resulting in foetal distress and hypoxia (Edmonds, 1954).

In 1994, with the application of the umbilical cord coiling index, Strong et al. proposed the prediction of adverse foetal effects (UCI) (Strong, Jarles, & Feldman, 1994). It was suggested that cord coiling may result from external hemodynamic stress or in-utero forces such as nuchal cords, foetal movement, distinct rates of umbilical vascular growth, morphology of muscle fibre, or genetics (Laat, Franx, & Nikkels, 2006; Lacro, Jones, & Benirschke, 1987). The Umbilical Coil is described as one full spiral of 360^o of umbilical vessels around each other. The coiling provides a flexible and powerful structure to the umbilical cord that offers resistance to external forces that might compromise blood flow to the foetus (Chitra, Sushanth, & Raghavan, 2012). Coiling is abnormal when UCI is less than the 10^th percentile (i.e., Hypocoiled cord) or if the UCI more than the 90^th percentile (i.e., Hypercoiled cord). UCI between 10^th and 90^th percentile is considered as Normocoiled cord.

Hypocoiled cord is been related to a higher risk of sudden decrease in blood flow due to kinking and marks the underlying intrinsic abnormal development. There is an association of higher prevalence of chromosomal and structural abnormalities, intrapartum FHR decelerations, chorioamnionitis, preterm birth, low Apgar scores, foetal demise, and operative delivery for foetal distress with hypocoiling (Sharma, Radhakrishnan, Manchanda, & Singh, 2018). Thus, abnormal intrinsic development is marked by such an association of abnormal coiling with foetal outcome. Hypercoiling of the cord in recent studies has been linked with intrapartum foetal asphyxia and acidosis, cord stenosis, and vascular thrombosis by predisposing to compression mediated reduction of flow in the fetoplacental circulation, which ultimately leads to foetal growth restriction (Sharma et al., 2018).

Accurate assessment and prenatal diagnosis for a better pregnancy outcome, is a result of proper understanding of normal UC development and the associated cord abnormalities. With the advent of new modern ultrasound techniques, abnormalities of the cord before birth have now become possible. Many studies related to this have been done in the western world but in India, very few studies have been conducted in assessing the foetal outcome based on the UC coiling. As defined in the second trimester with 2D ultrasound, there is currently no universal protocol or cut-off for the determination of the ultrasound umbilical coiling index. Quantifying postnatal UCI is of significance only if the concept can be applied in the antenatal period. Therefore, more prospective cohort studies are required which follow antenatal UCI in various trimesters till the postpartum period and assess its relation to maternal and foetal outcomes.

Method

The measurement of the UC through ultrasonography should be taken in the middle segment of the cord which floats in the amniotic fluid and should be avoided near the placental end or near the foetal end. The distance between the coils from the inner edge of an arterial or venous wall to the outer edge of the next coil along one side of the UC should be determined (Figure 1). As the pregnancy advances, there is a marked change in the morphology of the UC. The UC vessels transform easily from a predominantly parallel appearance to an usually twisted one during the first trimester. Similarly, an error in the calculation of cord coiling occurs in the third trimester of pregnancy due to a drop in the amount of amniotic fluid. Therefore 2^nd trimester, especially between 18-23 weeks of gestational age is the best chosen for the measurement of the antenatal umbilical coiling index (aUCI) along with the anomaly scan. Interestingly, other important events occur with the maturation of the UC morphology, such as the growth of the interval space and the presence of diastolic flow velocities in umbilical artery Doppler waveforms. UCI is calculated from the inner edge of the artery to the outer edge of the same artery along the side of the ipsilateral cord at the adjacent umbilical twist.

Google Scholar, PubMed, IndMed, and Cochrane Lib from January 1995 to May 2018 were the electronic databases used for the literature search for the present study. The publications were filtered and finally provided 39 articles of significance which were included for the review (Figure 2). Summary of various studies done in India examining the mean aUCI ± SD is mentioned in Table 1.

Anatomy and Embryology of Umbilical cord

Embryologically, the fascial margins of the umbilical defect are formed by the third week of foetal life, as the four folds of the somatopleure begin to fold inward. When the embryonic disc takes a cylindrical shape between 4 and 6 weeks of gestation, the UC starts to form at the lowest third of the embryo. Up-till the 10^th week of gestation, a sac herniation has the gut within, which starts developing from the proximal portion of the UC. The UC is greater in calibre at this time and is proportionately shorter than the embryo in-utero CRL and is thus unable to accommodate rotation over itself or around the developed embryo. The appearance of the cord is in the middle of the placenta, while the initial stalk develops at the centre of the implantation site. The intestines begin to return into the abdominal cavity in the coelom at the end of 10 weeks of gestation, leaving the proximal cord behind and the elongation of the cord starts along with the change in the position of the belly button in the middle third of the embryo. With the development of connective tissue of UC, i.e. Wharton’s Jelly, there is an elongation in the umbilical veins and arteries. By the 15th to 16th week of gestation, the allantois and the vitelline duct will regress. The manifestation of UC malformations is seen when all of these processes are defective (Hegazy, 2016).

Umbilical coiling index history

According to Chaurasia et al. the length of the cord is 0.3 cm long in 6 weeks of an embryo and has no twists, while, the length of the cord at the 8^th week of gestation is 1.8cm long (roughly in 50 days) when the first twist appears. Hence, it can be said that the cord is devoid of twists in the first 3 weeks of development, i.e., the 5^th, 6^th, and 7^th weeks of gestation. While the gain in the number of twists is not standardized, it appears that the average number of twists by the 9th week of gestation 33 is 6.7 for the left twists and 7.5 for the right twists. It is therefore remarked that, increase in the length of cord with the simultaneous appearance of the twist in a gradual fashion, probably results in gain in the number of the twists of the cord (Chaurasia & Agarwal, 1979). The intrauterine fetal activity is correlated with fetal well-being and also influences the length of the cord. Leonardo da Vinci claimed in the 15th century that the length of the UC is equal to the length of the child at each point of his age. Malpas and Symonds demonstrated in their study that the cord does not grow from one growing point, but grows uniformly over its length at each point. There is also a steady rise in the length of the cord with the increase in the primary helix pitch and not with the increase in the number of twists (Edmonds, 1954). Strong et al. in 1994, simplified by removing these directional scores and named it UCI (Strong et al., 1994). The umbilical coiling index is raised in polyhydramnios compared to oligohydramnios. This can be proved by the hypothesis of Edmond’s stating that there is an increase in the number of twists with the increase in the rotatory movement of the embryo (Figure 3).

Pathophysiology of coiling of the UC

The factors leading to the coiling of the cord are unknown. Various hypotheses have been put forward by various authors. They include (Table 2).

According to Roach, the 4 different types of muscle fibers in the wall of arteries are the main cause of coiling. The outer circular layer controls the flow, while the inner longitudinal layer closes the artery in the post-partum phase. The coiling occurs due to an intrinsic twist in the large coiling muscle which is comparable to the pitch of the coil itself, while the small coiling muscles help in the coiling of the arteries. The coiling of the cord is in the opposite direction to the direction of the fibres in the helical muscle when there is sufficient hydrostatic pressure. Reynolds proposed another mechanism explaining the benefits of UC coiling. According to him, the dynamic interaction between the umbilical arteries and the veins is due to their close association with each other. The arterial pressure pulses, pump to the adjacent umbilical vein carrying the fetal blood and thus leads to definitive increases and decreases in the venous pressure. Due to such an arrangement of the arteries and veins inside the UC, the arterial coils provide variable pressure to the vein along its length and thus results in a larger effect of the same. Degani also supported the above fact, by finding a linear relationship between the venous flow and the coiling index. In the case of overcoiling, due to the opposite nature of the increased turbulence to the arterial pressure pulse, compression of the vein occurs. By enlarge, the hypocoiled cords are more susceptible to acute kinking, and therefore marked the cessation of blood flow occurs, while, hypercoiled cords have greater resistance to flow through the coiled tube as suggested by the flow dynamic principles and literature studies. Hence, with the increase in the UCI, there is a potential benefit of increased umbilical blood flow probably due to the “Pulsometer effect”, but further increase in the coiling can predispose to compression mediated reduction of flow and possibility of development of fetoplacental vascular thrombosis can also occur (Figure 4).

Role of Wharton’s Jelly and Umbilical Coiling Index (UCI)

The amniotic membrane gradually surrounds the cord by 4 weeks post conception. The amnion sheaths the cord in the direction of the placenta with continuous enlargement of the amniotic cavity. Strength on the UC increased due to the presence of Wharton’s jelly around which is derived from mesenchyme and is formed by myofibroblast. Wharton’s jelly consists of collagen, hyaluronic acid, muscular fibers, and water. It has an angiogenic and metabolic role for umbilical circulation and provides mechanical support and structural protection to the umbilical vessels.

The osmotic environment of the Wharton’s jelly is of utmost importance. Evident swelling or shrinking of the cord results due to changes in osmolarity by 5 to 10 milli-osmol. The semi-solid gelatinous substance called Wharton jelly, has thyxotropic properties which liquefies due to pressure. This protective phenomenon was described as "spontaneous internal balloting" and was compared with the action of a concertina. Amongst twins, there is an absence of twists due to the reduction of the rotatory movement of the fetuses. By virtue of intrauterine crowding, twins have an increased frequency of restriction-related defects like short UC, so foetal rotation to either the left or right will occur less often in twins than in singletons.

It is also claimed that the UC with a 5th-10th centile diameter is an early marker for the delivery of a small infant for gestational age and thus intrapartum complications occur.

Furthermore, there is increased absence of twists among spontaneous stillbirths and abortus due to foetal defects which adversely affects cord twisting and leads to decreased foetal activity. The cord twist phenomenon is impaired due to reduced foetal activity as a result of intrinsic (foetal) and extrinsic (intrauterine constraint) causes (Table 4; Table 3).

Conclusions

The antenatal ultrasound diagnosis of umbilical cord abnormalities is used to triage patients as either being high- or low-risk during the antepartum and intrapartum period and may alert the obstetrician to an increased risk of a non-reassuring foetal status. Although UC is one of the most interesting human organ structures, it is one of the least studied. There is a huge opportunity for research in this field as what we are seeing is just the tip of an iceberg. We observed that neonatal outcomes can be enhanced with greater knowledge of the natural development of the umbilical cord, anatomy, and understanding of common cord defects, and can be applied in clinical practise by accurate antenatal ultrasound diagnosis and effective management. Thus, along with the existing literature, with proper training and technical advances, it is possible to prove as to how antenatal umbilical cord coiling index (aUCI) could be developed in the coming years as a promising non-invasive predictor of foetal wellbeing. The studies done previously in the present literature reflects the validity of measures used to predict the adverse perinatal outcome through the non-invasive investigations like ultrasonography which is typically favoured in antenatal period.

Funding Support

The authors declare that they have no funding support for this study.

Conflict of Interest

The authors declare that they have no conflict of interest for this study.