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. Developed from the same zygote as the fetus, the human umbilical cord 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.
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 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, and not of ordinary skin and connective tissue. The cord contains one vein, which carries oxygenated, nutrient-rich blood and two arteries that carry deoxygenated, nutrient depleted blood. The deoxygenated blood from the fetus passes very close to the mother's blood in the placenta, where oxygen and nutrients pass from mother's blood to the fetus' blood, and waste does the reverse. 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.
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.
Shortly after birth, upon exposure to temperature change, the gelatinous Wharton's Jelly substance undergoes a physiological change that collapses previous structure boundaries and in effect creates a natural clamp on the umbilical cord which halts placental blood return to the neonate, causing the cord to cease pulsation. This process will take as little as five minutes if left to proceed naturally.
A number of abnormalities can affect the umbilical cord, which can cause problems that affect both mother and child:
General obstetric practice introduces artificial clamping as early as 1 minute after birth of the child. This 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 2–3 weeks as it dries and then falls off.
Health effects of non-clamping of the cord and delayed umbilical severance are receiving attention in medical journals. Where cutting the cord is completely omitted (a practice called "lotus birth" by its advocates), the umbilical cord is wrapped up to within an inch of the newborn's belly, and the entire intact cord is allowed to dry like a sinew, which then falls off. A recent Cochrane Review studied effects of the timing of umbilical cord clamping. Infants whose cord clamping occurred later than 60 seconds after birth had a statistically higher risk of neonatal jaundice which required phototherapy. Infants with delayed clamping did have a higher hemoglobin level at 2 months, but this effect did not persist beyond 6 months of age.
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 (i.e. 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 somewhat controversial, with critics asserting that early cord blood withdrawal actually increases the likelihood of childhood disease. The Royal College of Obstetricians and Gynaecologists 2006 opinion states, "There is still insufficient evidence to recommend directed commercial cord blood collection and stem-cell storage in low-risk families."
In the future, cord blood-derived embryonic-like stem cells (CBEs) may also 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 phrase "cutting the umbilical cord" is used metaphorically to describe a child's breaking away from the parental home.