Inplacentalmammals, theumbilical cord (also called thenavel string,[1]birth cord orfuniculus umbilicalis) is a conduit between the developingembryo orfetus and theplacenta. Duringprenatal development, the umbilical cord is physiologically and genetically part of the fetus and (in humans) normally contains twoarteries (theumbilical arteries) and onevein (theumbilical vein), buried withinWharton's jelly. The umbilical vein supplies the fetus withoxygenated,nutrient-richblood from theplacenta. Conversely, the fetal heart pumps low-oxygen, nutrient-depleted blood through the umbilical arteries back to the placenta.
Cross-sectionalmicrograph of the human umbilical cord.H&E stain. Labelledallantoic duct is also known as theurachus.Vaginal ultrasonography of an embryo of agestational age of eight weeks and three days. The embryo is surrounded by the thin membranes of theamniotic sac, the umbilical cord is seen in the center, attaching the embryo to theplacenta.
The umbilical cord develops from and contains remnants of theyolk sac andallantois. It forms by the fifth week ofdevelopment, replacing the yolk sac as the source of nutrients for the embryo.[2] The cord is not directly connected to the mother's circulatory system, but instead joins theplacenta, which transfers materials to and from the maternal blood without allowing direct mixing. The length of the umbilical cord is approximately equal to thecrown-rump length of the fetus throughoutpregnancy. The umbilical cord in a full termneonate is usually about 50centimeters (20in) long and about 2 centimeters (0.75 in) in diameter. This diameter decreases rapidly within the placenta. The fully patent umbilical artery has two main layers: an outer layer consisting of circularly arranged smooth muscle cells and an inner layer which shows rather irregularly and loosely arranged cells embedded in abundantground substance stainingmetachromatic.[3] The smooth muscle cells of the layer are rather poorly differentiated, contain only a few tinymyofilaments and are thereby unlikely to contribute actively to the process of post-natal closure.[3]
The umbilical cord can be detected by ultrasound by six weeks of gestation and well-visualized by eight to nine weeks of gestation.[4]
The umbilicalcord lining is a good source of mesenchymal and epithelial stem cells. Umbilical cordmesenchymal stem cells (UC-MSC) have been used clinically to treat osteoarthritis, autoimmune diseases, and multiple other conditions. Their advantages include a better harvesting, and multiplication, and immunosuppressive properties that define their potential for use in transplantations. Their use would also overcome the ethical objections raised by the use ofembryonic stem cells.[5]
The umbilical cord containsWharton's jelly, a gelatinous substance made largely frommucopolysaccharides that protects the blood vessels inside. It contains one vein, which carries oxygenated, nutrient-rich blood to the fetus, and two arteries that carry deoxygenated, nutrient-depleted blood away.[6] Occasionally, only two vessels (one vein and one artery) are present in the umbilical cord. This is sometimes related to fetal abnormalities, but it may also occur without accompanying problems.
It is unusual for a vein to carry oxygenated blood and for arteries to carry deoxygenated blood (the only other examples being thepulmonary veins andarteries, connecting the lungs to the heart). However, this naming convention reflects the fact that the umbilical vein carries blood towards the fetus' heart, while the umbilical arteries carry blood away.
The blood flow through the umbilical cord is approximately 35 ml / min at 20 weeks, and 240 ml / min at 40weeks of gestation.[7] Adapted to the weight of the fetus, this corresponds to 115 ml / min / kg at 20 weeks and 64 ml / min / kg at 40 weeks.[7]
Forterms of location, theproximal part of an umbilical cord refers to the segment closest to the embryo or fetus in embryology and fetal medicine, and closest to the placenta in placental pathology, and opposite for thedistal part, respectively.[8]
The umbilical cord enters the fetus via theabdomen, at the point which (after separation) will become theumbilicus (belly button or navel). Within the fetus, the umbilical vein continues towards thetransverse fissure of theliver, where it splits into two. One of these branches joins with thehepatic portal vein (connecting to its left branch), which carries blood into the liver. The second branch (known as theductus venosus) bypasses the liver and flows into theinferior vena cava, which carries blood towards the heart. The two umbilical arteries branch from theinternal iliac arteries and pass on either side of theurinary bladder into the umbilical cord, completing the circuit back to the placenta.[9]
After birth, the umbilical cord stump will dry up and drop away by the time the baby is three weeks old.[10] If the stump still has not separated after three weeks, it might be a sign of an underlying problem, such as an infection or immune system disorder.[10]
In absence of external interventions, the umbilical cordoccludes physiologically shortly after birth, explained both by a swelling and collapse ofWharton's jelly in response to a reduction in temperature and byvasoconstriction of the blood vessels by smooth muscle contraction. In effect, a natural clamp is created, halting the flow of blood. In air at 18 °C, this physiological clamping will take three minutes or less.[11] Inwater birth, where the water temperature is close to body temperature, normal pulsation can be five minutes and longer.
Closure of the umbilical artery by vasoconstriction consists of multiple constrictions which increase in number and degree with time. There are segments of dilations with trapped uncoagulated blood between the constrictions before complete occlusion.[12] Both the partial constrictions and the ultimate closure are mainly produced by muscle cells of the outer circular layer.[3] In contrast, the inner layer seems to serve mainly as a plastic tissue which can easily be shifted in anaxial direction and then folded into the narrowing lumen to complete the closure.[3] The vasoconstrictive occlusion appears to be mainly mediated byserotonin[13][14] andthromboxane A2.[13] The artery in cords of preterm infants contracts more toangiotensin II andarachidonic acid and is more sensitive tooxytocin than in term ones.[14] In contrast to the contribution of Wharton's jelly, cooling causes only temporary vasoconstriction.[14]
Within the child, the umbilical vein and ductus venosus close up, and degenerate into fibrous remnants known as theround ligament of the liver and theligamentum venosum respectively. Part of each umbilical artery closes up (degenerating into what are known as themedial umbilical ligaments), while the remaining sections are retained as part of the circulatory system.
A number of abnormalities can affect the umbilical cord, which can cause problems that affect both mother and child:[15]
Umbilical cord compression can result from, for example, entanglement of the cord,[16] a knot in the cord,[16] or anuchal cord,[16] (which is the wrapping of the umbilical cord around the fetal neck)[17] but these conditions do not always cause obstruction of fetal circulation.
The umbilical cord is about to be cut with scissors via caesarean sectionUmbilical cord clampA day-old baby with the cord stump still attached.A 7 cm (2.75 in) long detached umbilical cord.
The cord can be clamped at different times; however, delaying the clamping of the umbilical cord until at least one minute after birth improves outcomes as long as there is the ability to treat the small risk ofjaundice if it occurs.[18] Clamping is followed by cutting of the cord, which is painless due to the absence ofnerves. The cord is extremely tough, like thicksinew, and so cutting it requires a suitably sharp instrument. While umbilical severance may be delayed until after the cord has stopped pulsing (one to three minutes after birth), there is ordinarily no significant loss of either venous or arterial blood while cutting the cord. Current evidence neither supports, nor refutes, delayed cutting of the cord, according to theAmerican Congress of Obstetricians and Gynecologists (ACOG) guidelines.
There are umbilical cord clamps which incorporate a knife. These clamps are safer and faster, allowing one to first apply the cord clamp and then cut the umbilical cord. After the cord is clamped and cut, the newborn wears a plastic clip on the navel area until the compressed region of the cord has dried and sealed sufficiently.
The length of umbilical left attached to the newborn varies by practice; in most hospital settings the length of cord left attached after clamping and cutting is minimal. In the United States, however, where the birth occurred outside of the hospital and anemergency medical technician (EMT) clamps and cuts the cord, a longer segment up to 18 cm (7 in) in length[19][20] is left attached to the newborn.
The remaining umbilical stub remains for up to ten days as it dries and then falls off.
ACochrane review in 2013 came to the conclusion that delayed cord clamping (between one and three minutes after birth) is "likely to be beneficial as long as access to treatment for jaundice requiring phototherapy is available".[21] In this review delayed clamping, as contrasted to early, resulted in no difference in risk of severe maternalpostpartum hemorrhage or neonatal mortality, and a lowApgar score. On the other hand, delayed clamping resulted in an increased birth weight of on average about 100 g, and an increasedhemoglobin concentration of on average 1.5 g/dL with half the risk of being iron deficient at three and six months, but an increased risk of jaundice requiringphototherapy.[21]
In 2012, the American College of Obstetricians and Gynecologists officially endorsed delaying clamping of the umbilical cord for 30–60 seconds with the newborn held below the level of the placenta in all cases of preterm delivery based largely on evidence that it reduces the risk of intraventricular hemorrhage in these children by 50%.[22][obsolete source] In the same committee statement, ACOG also recognize several other likely benefits for preterm infants, including "improved transitional circulation, better establishment of red blood cell volume, and decreased need for blood transfusion". In January 2017, a revised Committee Opinion extended the recommendation to term infants, citing data that term infants benefit from increased hemoglobin levels in the newborn period and improved iron stores in the first months of life, which may result in improved developmental outcomes. ACOG recognized a small increase in the incidence of jaundice in term infants with delayed cord clamping, and recommended policies be in place to monitor for and treat neonatal jaundice. ACOG also noted that delayed cord clamping is not associated with increased risk of postpartum hemorrhage.[23]
Several studies have shown benefits of delayed cord clamping: A meta-analysis[24] showed that delaying clamping of the umbilical cord in full-term neonates for a minimum of two minutes following birth is beneficial to the newborn in giving improvedhematocrit, iron status as measured byferritin concentration and stored iron, as well as a reduction in the risk of anemia (relative risk, 0.53; 95% CI, 0.40–0.70).[24] A decrease was also found in a study from 2008.[25] Although there is higher hemoglobin level at 2 months, this effect did not persist beyond 6 months of age.[26] Not clamping the cord for three minutes following the birth of a baby improved outcomes at four years of age.[27] A delay of three minutes or more in umbilical cord clamping after birth reduce the prevalence of anemia in infants.[28]
Negative effects of delayed cord clamping include an increased risk ofpolycythemia. Still, this condition appeared to be benign in studies.[24] Infants whose cord clamping occurred later than 60 seconds after birth had a higher rate ofneonatal jaundice requiringphototherapy.[26]
Delayed clamping is not recommended as a response to cases where the newborn is not breathing well and needs resuscitation. Rather, the recommendation is instead to immediately clamp and cut the cord and performcardiopulmonary resuscitation.[29] The umbilical cord pulsating is not a guarantee that the baby is receiving enough oxygen.[30]
Some parents choose to omit cord severance entirely, a practice called "lotus birth" or umbilical nonseverance. The entire intact umbilical cord is allowed to dry and separates on its own (typically on the 3rd day after birth), falling off and leaving a healed umbilicus.[31] TheRoyal College of Obstetricians and Gynaecologists has warned about the risks of infection as the decomposing placenta tissue becomes a nest for infectious bacteria such asStaphylococcus.[32] In one such case, a 20-hour old baby whose parents chose UCNS was brought to the hospital in an agonal state, was diagnosed withsepsis and required an antibiotic treatment for six weeks.[33][34]
As the umbilical vein is directly connected to the central circulation, it can be used as a route for placement of a venous catheter for infusion and medication. The umbilical vein catheter is a reliable alternative to percutaneous peripheral or central venous catheters or intraosseous canulas and may be employed in resuscitation or intensive care of the newborn.
From 24 to 34 weeks of gestation, when the fetus is typically viable, blood can be taken from the cord in order to test for abnormalities (particularly forhereditary conditions). This diagnosticgenetic test procedure is known aspercutaneous umbilical cord blood sampling.[35]
Some parents choose to have this blood diverted from the baby's umbilical blood transfer through early cord clamping and cutting, to freeze for long-term storage at acord blood bank should the child ever require the cord blood stem cells (for example, to replacebone marrow destroyed when treatingleukemia). This practice is controversial, with critics asserting that early cord blood withdrawal at the time of birth actually increases the likelihood of childhood disease, due to the high volume of blood taken (an average of 108ml) in relation to the baby's total supply (typically 300ml).[25] TheRoyal College of Obstetricians and Gynaecologists stated in 2006 that "there is still insufficient evidence to recommend directed commercial cord blood collection and stem-cell storage in low-risk families".[36]
TheAmerican Academy of Pediatrics has stated that cord blood banking for self-use should be discouraged (as most conditions requiring the use of stem cells will already exist in the cord blood), while banking for general use should be encouraged.[37] In the future, cord blood-derived embryonic-like stem cells (CBEs) may be banked and matched with other patients, much like blood and transplanted tissues. The use of CBEs could potentially eliminate the ethical difficulties associated withembryonic stem cells (ESCs).[38]
While the American Academy of Pediatrics discourages private banking except in the case of existing medical need, it also says that information about the potential benefits and limitations of cord blood banking and transplantation should be provided so that parents can make an informed decision.
In the United States, cord blood education has been supported by legislators at the federal and state levels. In 2005, the National Academy of Sciences published anInstitute of Medicine (IoM) report which recommended that expectant parents be given a balanced perspective on their options for cord blood banking. In response to their constituents, state legislators across the country are introducing legislation intended to help inform physicians and expectant parents on the options for donating, discarding or banking lifesaving newborn stem cells. Currently 17 states, representing two-thirds of U.S. births, have enacted legislation recommended by the IoM guidelines.
The use of cord blood stem cells in treating conditions such as brain injury[39] and Type 1 Diabetes[40] is already being studied in humans, and earlier stage research is being conducted for treatments of stroke,[41][42] and hearing loss.[43]
Cord blood stored with private banks is typically reserved for use of the donor child only. In contrast, cord blood stored in public banks is accessible to anyone with a closely matching tissue type and demonstrated need.[44] The use of cord blood from public banks is increasing. Currently it is used in place of a bone marrow transplant in the treatment of blood disorders such as leukemia, with donations released for transplant through one registry, Netcord.org,[45] passing 1,000,000 as of January 2013. Cord blood is used when the patient cannot find a matching bone marrow donor; this "extension" of the donor pool has driven the expansion of public banks.
The umbilical cord in some mammals, including cattle and sheep, contains two distinct umbilical veins. There is only one umbilical vein in the human umbilical cord.[46]
In some animals, the mother will gnaw through the cord, thus separating the placenta from the offspring. The cord along with the placenta is often eaten by the mother, to provide nourishment and to dispose of tissues that would otherwise attract scavengers or predators.[citation needed] Inchimpanzees, the mother leaves the cord in place and nurses her young with the cord and placenta attached until the cord dries out and separates naturally, within a day of birth, at which time the cord is discarded. (This was first documented by zoologists in the wild in 1974.[47])
The term "umbilical cord" or just "umbilical" has also come to be used for other cords with similar functions, such as the hose connectingsurface-supplied divers to their surface supply of air and/or heating, orspace-suitedastronauts to their spacecraft. Engineers sometimes use the term to describe a complex or critical cable connecting a component, especially when composed of bundles of conductors of different colors, thickness and types, terminating in a single multi-contact disconnect.
^abcdMeyer WW, Rumpelt HJ, Yao AC, Lind J (July 1978). "Structure and closure mechanism of the human umbilical artery".Eur. J. Pediatr.128 (4):247–59.doi:10.1007/BF00445610.PMID668732.S2CID37516644.
^Yao AC, Lind J, Lu T (1977). "Closure of the human umbilical artery: a physiological demonstration of Burton's theory".Eur. J. Obstet. Gynecol. Reprod. Biol.7 (6):365–8.doi:10.1016/0028-2243(77)90064-8.PMID264063.
^abQuan A, Leung SW, Lao TT, Man RY (December 2003). "5-hydroxytryptamine and thromboxane A2 as physiologic mediators of human umbilical artery closure".J. Soc. Gynecol. Investig.10 (8):490–5.doi:10.1016/S1071-5576(03)00149-7.PMID14662162.
^abcWhite RP (January 1989). "Pharmacodynamic study of maturation and closure of human umbilical arteries".Am. J. Obstet. Gynecol.160 (1):229–37.doi:10.1016/0002-9378(89)90127-0.PMID2912087.
^Committee on Obstetric Practice, American College of Obstetricians and, Gynecologists (December 2012). "Committee Opinion No.543: Timing of umbilical cord clamping after birth".Obstetrics and Gynecology.120 (6):1522–6.doi:10.1097/01.aog.0000423817.47165.48.PMID23168790.
^abcHutton EK, Hassan ES (March 2007). "Late vs early clamping of the umbilical cord in full-term neonates: systematic review and meta-analysis of controlled trials".JAMA.297 (11):1241–52.doi:10.1001/jama.297.11.1241.PMID17374818.
^abExamination of the Newborn & Neonatal Health: A Multidimensional Approach, p. 116-117
^ab"Effect of timing of umbilical cord clamping of term infants on maternal and neonatal outcomes."Cochrane Database Syst Rev. 2008; (2):CD004074
^Vendrame M, et al. (2005). "Anti-inflammatory effects of human cord blood cells in a rat model of stroke".Stem Cells Dev.14 (5):595–604.doi:10.1089/scd.2005.14.595.PMID16305344.
