Year: 2007
Type: article
Abstract: In this issue, Cromi et al. report that sonographic assessment of cross-sectional umbilical cord area can predict fetal macrosomia1. This is the latest of numerous recent studies that have investigated the possible relationship between umbilical cord characteristics and fetal outcome, including cord thickness, but particularly in relation to umbilical cord coiling. The phenomenon of coiling of the vessels within the umbilical cord in a regular helical pattern has long been recognized, but its precise underlying causative mechanism and presumed developmental benefit remain uncertain. In the pathology literature, focal tight twisting of the umbilical cord has been associated with a proportion of intrauterine deaths, although the clinical significance of this association has remained unclear. Some have suggested that focal coiling is a consequence, rather than a cause, of intrauterine demise, whereas others have postulated that the overcoiled cord may in itself be a contributory factor to fetal death2. Numerous investigators have recently attempted to study further the relationship between the coiling pattern of the umbilical cord and a range of pregnancy complications, with initial studies focusing on the association between undercoiled cords and adverse outcomes3, 4. Several subsequent larger pathological studies, based on examination of the delivered placenta, have confirmed these associations and, in conjunction with the recognition that umbilical cord coiling could be determined antenatally by ultrasound examination5, 6, additional reports have suggested a relationship between extremes of abnormal cord coiling and diverse adverse pregnancy outcomes3-17. This Opinion aims to place such findings in context and provide a summary of the current state of knowledge regarding cord coiling and pregnancy outcome. The generally accepted method of assessing the degree of the umbilical cord coiling is by calculation of the umbilical coiling index (UCI), defined as the number of complete coils per centimeter length of cord3. Using this criterion, studies to date have been remarkably consistent in reporting of the normal UCI, which is around 0.2 in the postpartum setting following examination of the delivered placenta and umbilical cord (pUCI) and 0.4 when determined antenatally by sonography (aUCI)18, 19. This apparent discrepancy, with increased coiling identified in utero, presumably is simply a reflection of the fact that the umbilical vessels are distended with fetal blood antenatally and this simple mechanical distension will result in a tighter apparent coiling of helical vessels. In order to compare the results of various studies, undercoiled/hypocoiled cords have been defined as those with an UCI < 10th centile, whereas overcoiled/hypercoiled cords are defined as those with an UCI > 90th centile. Such studies have determined that the degree of cord coiling can be reliably identified sonographically from the first trimester and that the UCI is related to gestation and the site of sampling (coiling being greater towards the fetal end of the cord)20. The mechanism by which physiological coiling occurs still, however, remains undetermined, with speculation that it may be related to early fetal activity and hemodynamic factors (supported by the finding of increased coiling in the recipient twin in twin-to-twin transfusion syndrome), or other anatomical issues such as the presence of Roach muscle21, 22. Although an association between the aUCI and pUCI is present, which is not surprising as both are a measure of the same phenomenon, antenatal sonographic determination of UCI has a low sensitivity (40%) for predicting the presence of undercoiling or overcoiling as determined by postnatal cord examination16. Therefore, it appears that the antenatal and postnatal coiling indices may not represent measurements of exactly the same process (see previous comment regarding vascular distension in utero); some cases that appear overcoiled or undercoiled on antenatal examination are within the normal range after delivery, and vice versa. Nevertheless, despite these findings of apparent discordance, numerous studies have reported a range of adverse outcomes in association with abnormal cord coiling, detected either prenatally or postnatally (Tables 1 and 2). Essentially, it appears that hypocoiled cords are predominantly associated with an increased frequency of intrauterine death, low Apgar score, the presence of fetal congenital anomalies such as trisomies, and other abnormalities of placental development such as velamentous insertion and presence of a single umbilical artery. Such undercoiled cords are not generally associated with intrauterine growth restriction, fetal acidosis or asphyxia. Hypercoiled cords also appear to be associated with fetal abnormalities such as trisomies and single umbilical artery. However, in contrast to undercoiled cords, they are not associated with an increased likelihood of intrauterine death or low Apgar score, but rather show an association with intrauterine growth restriction, fetal acidosis and asphyxia. Although there are differences in the precise associations and their magnitude, these findings appear fairly consistent across studies, both antenatal and postnatal. Given that such data exist, the question arises as to whether these are simply associations with no pathophysiological significance or whether the cord coiling pattern can lead directly to adverse pregnancy outcome. Mechanisms by which this could be mediated include the possibility that undercoiled cords may be more susceptible to acute kinking and therefore abrupt and marked cessation of blood flow; in the case of hypercoiled cords, flow dynamic principles and studies suggest that flow through a coiled tube should be associated with greater resistance to flow than through a straight tube23. However, a study examining the relationship between aUCI and Doppler flow indices reported that increasing aUCI is in reality related to increased umbilical vein flow and reduced umbilical artery resistance24. Furthermore, mechanical data indicate no difference in the effect of kinking according to UCI, but clearly demonstrate that greater cord coiling is associated with increased susceptibility to external compression-related reduction of flow25, 26. Therefore, it appears that increasing UCI may be potentially beneficial by increasing umbilical blood flow, probably as a result of localized pulsometer effects, but further increased coiling may predispose to compression-mediated flow reduction and possible predisposition to the development of fetoplacental vascular thromboses17. In summary, it can be concluded that the majority of pregnancies with overcoiled or undercoiled umbilical cords, as determined by assessment of UCI, have a normal outcome. However, there appear to be consistent and clinically significant associations between abnormal coiling and a range of adverse pregnancy outcomes. Hypocoiled cords are mainly associated with intrauterine death, fetal anomalies and abnormal insertion; they are therefore likely to represent a marker of underlying intrinsic abnormal development, and are also possibly associated with an increased risk of acute reduction in blood flow due to kinking. Hypercoiled cords, in addition to being possible markers of abnormal development, are associated with other complications including intrauterine growth restriction, fetal acidosis and asphyxia. No conclusive hemodynamic mechanism to explain these associations has yet been determined but such overcoiled cords may be at increased risk of causing embarrassment to fetoplacental flow in association with external compression, such as nuchal coiling (Figure 1). Further studies are necessary to explore such mechanisms. At present, fetal medicine practitioners and perinatal pathologists should continue to record and investigate clinical features associated with abnormal cord coiling patterns but, on the basis of currently available data, should remain cautious in assigning causality of any adverse outcome purely on the basis of the UCI. Schematic illustrating possible pathophysiological relationships between reported associations of features in pregnancies with undercoiled and overcoiled umbilical cords.
DOI: 10.1002/uog.5180
Author: N. J. Sebire
Institution: Great Ormond Street Hospital
Language: English
Subfield: Obstetrics and Gynecology
Field: Medicine
Domain: Health Sciences
Open Access status: bronze
OpenAlex ID: W2094086451