Review article The development, structure and blood flow within the umbilical cord with particular reference to the venous system Abstract Jacqueline Spurway1 The umbilical cord is a helical and tubular blood conduit connecting the foetus to the placenta. The AMS umbilical cord achieves its final form by the 12th week of gestation and normally contains two arteries and a single vein, all embedded in Wharton’s jelly. The structure of the umbilical cord receives only a Patricia Logan2 cursory glance during many obstetric ultrasound examinations: with imaging limited to documenting PhD the number of vessels within the cord and the insertion sites at the foetus and placenta. Extensive 3 research into blood flow characteristics of the umbilical cord arteries has been undertaken and is now Sokcheon Pak widely applied in contemporary ultrasound practice. In contrast, investigation of umbilical vein blood PhD flow is only instigated in instances of foetal compromise when the spectral waveform of the ductus 1Medical Imaging venosus and pulsations in the vein are scrutinised. The current level of ultrasound imaging of the Department umbilical vein demonstrates a lack of appreciation and knowledge about a structure that is crucial to Orange Health Service sustaining foetal life. Orange The goal of this review is to increase awareness of the importance of the umbilical cord. In addition, New South Wales this review will provide an information platform for undertaking and critically analysing research into Australia the umbilical cord by providing a summary of cord embryology, structure, foetal venous circulation and mechanisms of blood flow within the umbilical cord vein. 2School of Biomedical Science Keywords: umbilical cord, umbilical cord vein, venous blood flow. Charles Sturt University Dubbo New South Wales Introduction the 4th to 8th weeks of gestation (calculated from Australia Te umbilical cord provides the pathway for the frst day of the last menstrual cycle) by the unhindered blood transport from the placenta expanding amnion enveloping tissue from the 3School of Biomedical to the foetus and vice versa. Aristotle (384–322 body stalk, the omphalomesenteric duct and Science 2 BC) originally identifed the umbilical cord as the the umbilical coelom. Blood fow is established Charles Sturt University connection between the mother and unborn child.1 within the umbilical cord by the end of the 5th Tis article is the consequence of a systemic week of gestation.3 Bathurst review of established texts, peer reviewed articles Coursing through the body stalk are two New South Wales and credible websites which was undertaken to umbilical arteries, two umbilical veins and the Australia develop a knowledge base about the development, allantois. Initially, the lef and right umbilical structure and blood movement within the foetal arteries are caudal continuations of the primitive Correspondence to email venous system in preparation for undertaking dorsal aortae, but afer several revisions fnally jacqueline.spurway@ research into quantifable aspects of the umbilical arise from the internal iliac arteries.4 Afer birth, gwahs.health.gov.nsw.au cord vein. the proximal portions of the intra-abdominal Te following pages review the development, umbilical arteries become the internal iliac structure, foetal venous circulation and and superior vesical arteries, while the distal mechanisms of blood fow within the umbilical portions are obliterated and form the medial cord vein with the aim of fostering an increased umbilical ligaments.5 Te umbilical veins arise appreciation of a structure that is ofen neglected from a convergence of venules that drain the during routine obstetric ultrasound examinations extra-embryonic allantois.4 Obliteration of and to provide an information platform for the right umbilical vein by the end of the 6th research into the foetal venous network. week of gestation6 results in a single persisting lef umbilical vein. Obliteration of the intra- Umbilical cord embryology abdominal umbilical vein at birth produces a Te rudimentary umbilical cord is formed during hepatic remnant termed the ligamentum teres.5 AJUM August 2012 15 (3) 97 Spurway, et al. Figure 1: A cross-sectional image of a postpartum umbilical cord showing the major structures (Hill M. UNSW Embryology. Placenta Histology. [Internet] 2010 [updated 2010 26th May; cited 2011 7th March]; Available from: http://embryology.med. unsw.edu.au/notes/placen- ta5.htm). Te allantois arises as a diverticulum of the yolk sac extending less than 30 cm considered short.14 Excessively long cords may from the early foetal bladder into the body stalk and contributes be associated with prolapse, looping of the cord around the foetal to umbilical vessel development. Regression of the allantois neck,15 entanglement, distress and foetal demise.14 On the other occurs between the 6th and 8th weeks of gestation7 and the hand very short cords may be associated with delayed foetal remnant is located between the two arteries within the cord. Te descent, premature placental separation,15 growth restriction, intra-abdominal remnant of the allantois involutes to a thick congenital abnormalities, foetal distress and demise.14 tube termed the urachus or the median umbilical ligament.8 In a study involving 368 healthy pregnant women the average In addition to the body stalk, the expanding amnion diameter of the umbilical cord was shown to range from 3.19 envelopes the umbilical coelom and the omphalomesenteric ± 0.40 mm at 10 weeks gestation, to a peak of 16.72 ± 2.57 mm duct. Te umbilical coelom connects the extra-embryonic at 33–35 weeks and then decline to 14.42 ± 1.50 mm at 42 weeks coelom with the intra-embryonic coelom and regresses by the gestation.13 Te decline in cord diameter towards term is attributed 12th week of gestation. Te omphalomesenteric or vitelline duct to a reduction in the water content of Wharton’s jelly16 which is a connects the primitive intestines with the yolk sac and usually mucous connective tissue surrounding the umbilical vessels.10 regresses between the 7th and 10th weeks of gestation, leaving Te diameter of the umbilical cord vein increases from 4.1 a solid, intra-abdominal cord from the ileum to the umbilicus.9 mm at 20 weeks to 8.3 mm at 38 weeks gestation.17 Togni, et al.18 demonstrated an increase in the cross-sectional area of Umbilical cord structure the umbilical cord vein from 28 mm2 at 24 weeks gestation to Te fully developed umbilical cord normally contains two a maximum of approximately 58mm2 between 34–38 weeks, umbilical arteries, one umbilical vein, the remnant of the followed by a slight decline from the 39th week. Another variation allantois all embedded in Wharton’s jelly and surrounded by a in the umbilical cord vein is a decrease in the diameter of vessel single layer of amnion10,11 as shown in Figure 112 and Figure 2a. by approximately 1mm between the placental and foetal ends.19 At term the umbilical cord has a average length of 50–60 Te area of the umbilical cord vein is approximately 30% larger cm.13 Normal cord length can range from 30 cm to 100 cm, with than the combined areas of the arteries and as such the velocity in 98 AJUM August 2012 15 (3) The development, structure and blood flow within the umbilical cord with particular reference to the venous system a Figure 2: B mode imag- es of a transverse sec- tion of the umbilical cord with (a) showing a normal three-vessel cord and (b) showing a b single umbilical artery ( t ) and vein ( t ). the vein is approximately half the velocity in either artery,20 with artery (Figure 2b), with the lef umbilical artery being absent UIFWFMPDJUZJOUIFVNCJMJDBMDPSEWFJOSBOHJOHGSPNmDNTT21 more frequently.23 A single umbilical artery may be associated Te umbilical cord arteries and vein are unlike their with aneuploid foetuses, or with intrauterine growth restriction counterparts in the remainder of the foetal body as the umbilical and renal anomalies in euploid foetuses.23 cord vein transports oxygenated blood to the foetal heart22 while Located within 3 cm of the cord insertion into the placenta the arteries return oxygen-depleted blood to the placenta. Te surface14 there is a 1.5–2 cm long shunt between the umbilical walls of the umbilical cord artery lack an internal and external cord arteries, termed the Hyrtl anastomosis.24 Te functions elastic lamina and the adventitia found in other arteries is of the Hyrtl anastomosis are to equalise pressure between the replaced by mucous connective tissue. Te umbilical cord vein umbilical arteries before they enter the placenta and to act as a has a thickened muscularis layer with intermingling circular, safety valve in case of placental compression or blockage of an longitudinal and oblique smooth muscle fbres as well as an umbilical artery.25 internal elastic lamina.10 Te two umbilical arteries commonly form a cylindrical helix In some cases, one of the umbilical cord arteries may undergo around the umbilical vein (Figure 3a). Te normal umbilical cord atresia, aplasia or agenesis resulting in a single umbilical cord has one coil per 5 cm of cord length.23 Te umbilical cord may AJUM August 2012 15 (3) 99 Spurway, et al. a Figure 3: Longitudinal images of the umbilical cord with (a) showing paired arteries ( t ) spiralling around the umbilical vein ( t ) and (b) showing the b vein looping around central coiled arteries. develop up to 40 spirals and there may be straight portions or with suboptimal pregnancy outcomes including increased reversal of spiral direction in diferent segments.26 In most cases incidence of interventional deliveries, higher cord pH and heart the umbilical arteries twist over the vein, however, in 4.2% of cases rate disturbances.27 However, mechanical testing has proven that the vein may twist around straight or hypocoiled arteries23 (Figure uncoiled cords sufer no more ill efects of external compression, 3b).