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Umbilical cord
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In placental mammals, the umbilical cord (also called the birth cord or funiculus umbilicalis) is the connecting cord from the developing embryo or fetus to the placenta. During prenatal development, the umbilical cord comes from the same zygote as the fetus and (in humans) normally contains two arteries (the umbilical arteries) and one vein (the umbilical vein), buried within Wharton's jelly.
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In placental mammals, the umbilical cord (also called the birth cord or funiculus umbilicalis) is the connecting cord from the developing embryo or fetus to the placenta. During prenatal development, the umbilical cord comes from the same zygote as the fetus and (in humans) normally contains two arteries (the umbilical arteries) and one vein (the umbilical vein), buried within Wharton's jelly. The umbilical vein supplies the fetus with oxygenated, nutrient-rich blood from the placenta. Conversely, the umbilical arteries return the deoxygenated, nutrient-depleted blood.
Physiology in humans
Development and composition
The umbilical cord develops from and contains remnants of the yolk sac and allantois (and is therefore derived from the same zygote as the fetus). 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 umbilical cord in a full term neonate is usually about 50 centimetres (20 in) long and about 2 centimetres (0.75 in) diameter. This diameter decreases rapidly within the placenta.
The umbilical cord is composed of Wharton's jelly, a gelatinous substance made largely from mucopolysaccharides. 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 (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 the pulmonary veins and arteries, connecting the lungs to the heart). However, this naming convention reflects the fact that the umbilical vein carries blood towards the fetus's heart, whilst the umbilical arteries carry blood away.
Connection to fetal circulatory system
The umbilical cord enters the fetus via the abdomen, at the point which (after separation) will become the umbilicus (or navel). 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 (connecting to its left branch), which carries blood into the liver. The second branch (known as the ductus venosus) allows the majority of the incoming blood (approximately 80%) to bypass the liver and flow via the left hepatic vein 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 before joining the umbilical cord.
Postnatal detachment
Shortly after birth, the reduction in temperature starts a physiological process which causes the Wharton's Jelly to swell and collapse the blood vessels within. This, in effect, creates a natural clamp, halting the flow of blood. This physiological clamping will take as little as five minutes if left to proceed naturally.
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 (degenerating into what are known as the medial umbilical ligaments), whilst 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:
Medical protocols and procedures
Clamping and cutting
General hospital-based obstetric practice introduces artificial clamping as early as 1 minute after the birth of the child. In birthing centers, this may be delayed by 5 minutes or more, or omitted entirely. Clamping is followed by cutting of the cord, which is painless due to the lack of any nerves. The cord is extremely tough, like thick sinew, and so cutting it requires a suitably sharp instrument. Provided that umbilical severance occurs after the cord has stopped pulsing (5-20 minutes after birth), there is ordinarily no significant loss of either venous or arterial blood while cutting the cord.
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 remaining umbilical stub remains for up to 7–10 days as it dries and then falls off.
Early versus delayed clamping
The health implications of early versus delayed cord clamping are receiving attention in medical journals.
Advocates of delayed clamping state that premature clamping increases the risk of ischemia and hypovolaemic shock. They also propose that umbilical cord blood contains the full complement of clotting factors required at birth, and that premature clamping reduces the amount delivered, requiring a protocol of Vitamin K administration. A recent analysis of attended home births over a 6-year period reported that none of the infants experienced adverse outcomes as a result of delayed cord clamping.
A recent Cochrane Review studied the effects of the timing of umbilical cord clamping in hospitals. Infants whose cord clamping occurred later than 60 seconds after birth had a statistically higher risk of neonatal jaundice requiring phototherapy. Whilst infants with delayed clamping did have a higher hemoglobin level at 2 months, this effect did not persist beyond 6 months of age. Conversely, a recent randomized, controlled trial noted in the 2008 Examination of the Newborn & Neonatal Health compared the timing of cord clamping on the newborn venous hematocrit and reported an increase in anemia in the infants whose cords were clamped immediately.
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 like a sinew, which then separates naturally (typically on the 3rd day after birth), falling off and leaving a healed umbilicus.
Storage of cord blood
Recently, it has been discovered that the blood within the umbilical cord, known as cord blood, is a rich and readily available source of primitive, undifferentiated stem cells (of type CD34-positive and CD38-negative). These cord blood cells can be used for bone marrow transplant.
Some parents have chosen to have this blood diverted from the baby's umbilical blood transfer through early cord clamping and cutting, to freeze for long-term (and costly) storage at a cord blood bank should the child ever require the cord blood stem cells (for example, to replace bone marrow destroyed when treating leukemia). 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). 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 (as most conditions requiring the use of stem cells will already exist in the cord blood), whilst banking for general use should be encouraged. 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 with embryonic stem cells (ESCs).
The umbilical cord in other mammals
Anatomy
The umbilical cord in some mammals contains two distinct umbilical veins, rather than just one (as is the case for humans). 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 (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. 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. (This was first documented by zoologists in the wild in 1974.)
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 a surface-supplied diver to his surface supply of air and/or heating, or a space-suited astronaut to his 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.
The phrase "cutting the umbilical cord" is used metaphorically to describe a child's breaking away from the parental home, or any person severing a dependency on another person.
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