In placental mammals, the umbilical cord is a conduit between the developing embryo or
fetus and the placenta. During prenatal development, the umbilical cord is physiologically and
genetically part of the fetus and normally contains two arteries and one vein, buried
within Wharton's jelly. The umbilical vein supplies the fetus with oxygenated, nutrient-rich
blood from the placenta. Conversely, the fetal heart pumps deoxygenated, nutrient-depleted
blood through the umbilical arteries back to the placenta.
Physiology in humans Development and composition
The umbilical cord develops from and contains remnants of the yolk sac and allantois. It
forms by the fifth week of fetal development, replacing the yolk sac as the source of nutrients
for the fetus. The cord is not directly connected to the mother's circulatory system, but instead
joins the placenta, which transfers materials to and from the mother's blood without allowing
direct mixing. The length of the umbilical cord is approximately equal to the crown-rump
length of the fetus throughout pregnancy. The umbilical cord in a full term neonate
is usually about 50 centimeters long and about 2 centimeters 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 abundant ground substance staining
metachromatic. The smooth muscle cells of the layer are rather poorly differentiated,
contain only a few tiny myofilaments and are thereby unlikely to contribute actively to
the process of postnatal closure. The umbilical cord contains Wharton's jelly,
a gelatinous substance made largely from mucopolysaccharides which 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. Occasionally, only two vessels
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. However, this naming convention reflects the fact that the umbilical vein carries blood
towards the fetus's 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 40 weeks of gestation. 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.
Connection to fetal circulatory system The umbilical cord enters the fetus via the
abdomen, at the point which will become the umbilicus. Within the fetus, the umbilical
vein continues towards the transverse fissure of the liver, where it splits into two. One
of these branches joins with the hepatic portal vein, which carries blood into the liver.
The second branch bypasses the liver and flows into the inferior vena cava, which carries
blood towards the heart. The two umbilical arteries branch from the internal iliac arteries,
and pass on either side of the urinary bladder into the umbilical cord, completing the circuit
back to the placenta. Physiological postnatal occlusion
In absence of external interventions, the umbilical cord occludes physiologically shortly
after birth, explained both by a swelling and collapse of Wharton's jelly in response
to a reduction in temperature and by vasoconstriction 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. In water birth, where the water temperature
is close to body temperature, normal pulsation can be 5 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 dilatations with trapped uncoagulated
blood between the constrictions before complete occlusion. Both the partial constrictions
and the ultimate closure are mainly produced by muscle cells of the outer circular layer.
In contrast, the inner layer seems to serve mainly as a plastic tissue which can easily
be shifted in an axial direction and then folded into the narrowing lumen to complete
the closure. The vasoconstrictive occlusion appears to be mainly mediated by 5-hydroxytryptamine
and thromboxane A2. The artery in cords of preterm infants contracts more to angiotensin
II and arachidonic acid and is more sensitive to oxytocin than in term ones. In contrast
to the contribution of Wharton's jelly, cooling causes only temporary vasoconstriction.
Within the child, the umbilical vein and ductus venosus close up, and degenerate into fibrous
remnants known as the round ligament of the liver and the ligamentum venosum respectively.
Part of each umbilical artery closes up, while the remaining sections are retained as part
of the circulatory system. Problems and abnormalities
A number of abnormalities can affect the umbilical cord, which can cause problems that affect
both mother and child: Umbilical cord compression can result from,
for example, entanglement of the cord, a knot in the cord, or a nuchal cord, but these conditions
do not always cause obstruction of fetal circulation. Velamentous cord insertion
Single umbilical artery Umbilical cord prolapse
Vasa praevia Medical protocols and procedures
Clamping and cutting
The cord can be clamped at different times; however delaying the clamping of the umbilical
cord until one minute after birth improves outcomes as long as there is the ability to
treat jaundice if it occurs. Clamping is followed by cutting of the cord, which is painless
due to the absence of nerves. The cord is extremely tough, like thick sinew, and so
cutting it requires a suitably sharp instrument. While umbilical severance may be delayed until
after the cord has stopped pulsing, 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 American College of Obstetricians & Gynecologists
guidelines. There are umbilical cord clamps which combine
the cord clamps with the 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 some counties in the United States, however,
where the birth occurred outside of the hospital and an emergency medical technician clamps
and cuts the cord, a longer segment up to 18 cm in length is left attached to the newborn.
The remaining umbilical stub remains for up to 10 days as it dries and then falls off.
Early versus delayed clamping A Cochrane review in 2013 came to the conclusion
that delayed cord clamping is "likely to be beneficial as long as access to treatment
for jaundice requiring phototherapy is available". In this review delayed clamping, as contrasted
to early, resulted in no difference in risk of severe maternal postpartum hemorrhage or
neonatal mortality, low Apgar score. On the other hand, delayed clamping resulted in an
increased birth weight of on average about 100 g, and an increased hemoglobin 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 requiring phototherapy.
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%. 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", as well as several for full term infants, including
improved iron stores and increased blood volume, however it stopped short of endorsing delayed
clamping for term infants due to a lack of evidence showing that these improved final
outcomes or outweighed the increased risk of polycythemia or hyperbilirubinemia associated
with delayed clamping. Several studies have shown benefits of delayed
cord clamping: A meta-analysis showed that delaying clamping of the umbilical cord in
full-term neonates for a minimum of 2 minutes following birth is beneficial to the newborn
in giving improved hematocrit, iron status as measured by ferritin concentration and
stored iron, as well as a reduction in the risk of anemia. A decrease was also found
in a study from 2008. Although there is higher hemoglobin level at 2 months, this effect
did not persist beyond 6 months of age. Negative effects of delayed cord clamping
include an increased risk of polycythemia. Still, this condition appeared to be benign
in studies. Infants whose cord clamping occurred later than 60 seconds after birth had a higher
rate of neonatal jaundice requiring phototherapy. 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 perform cardiopulmonary
resuscitation. The umbilical cord pulsating is not a guarantee that the baby is receiving
enough oxygen. Umbilical nonseverance
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, falling off and leaving a healed umbilicus.
Umbilical cord catheterization 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.
Storage of cord blood
The blood within the umbilical cord, known as cord blood, is a rich and readily available
source of primitive, undifferentiated stem cells. These cord blood cells can be used
for bone marrow transplant. 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 a cord blood bank should the child ever require the cord blood
stem cells. 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 in relation to the baby's total supply. The Royal 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". The American Academy of Pediatrics has stated
that cord blood banking for self-use should be discouraged, while banking for general
use should be encouraged. In the future, cord blood-derived embryonic-like stem cells 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 with embryonic
stem cells. 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 an Institute of Medicine
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 and Type 1 Diabetes is already being studied in humans,
and earlier stage research is being conducted for treatments of stroke, and hearing loss.
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. 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
[3], 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 other mammals
Anatomy The umbilical cord in some mammals contains
two distinct umbilical veins, rather than just one. Examples include cows and sheep.
Cord disposal In some animals, the mother will gnaw through
the cord, thus separating the placenta from the offspring. It is often eaten by the mother,
to provide nourishment and to dispose of tissues that would otherwise attract scavengers or
predators. In chimpanzees, the mother focuses no attention on umbilical severance, instead
nursing her baby with cord, placenta, and all, until the cord dries and separates within
a day of birth, at which time the cord is discarded.
Other uses for the term "umbilical cord" The term "umbilical cord" or just "umbilical"
has also come to be used for other cords with similar functions, such as the hose connecting
surface-supplied divers to their surface supply of air and/or heating, or space-suited astronauts
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.
Cancer-causing toxins in human umbilical cords In multiple American and international studies,
cancer-causing chemicals have been found in the blood of umbilical cords. These originate
from certain plastics, computer circuit boards, fumes and synthetic fragrances among others.
Over 300 toxic chemicals have been found, including Bisphenol A, tetrabromobisphenol
A, Teflon-relative perfluorobutanoic acid, galaxolide and tonalide among others.[4] Blacks,
Hispanics, and Asians, and the poor tend to have higher rates [5].
Additional images
See also Umbilical line
Americord Registry LifebankUSA
References
External links