umbilical cord

umbilical cord When the embryo first embeds itself in the lining of the uterus it is small enough for adequate exchange of dissolved gases, nutrients, and waste across its surface. Then as part of very early growth and reshaping, a short stalk is formed of its own tissue. Blood vessel loops grow into this stalk along with the development of the heart and circulation. The stalk lengthens as the fetus develops within its amniotic sac, and at the uterine end the blood vessels become part of the developing placenta. Thus the fetus grows its own umbilical cord, containing its own blood vessels: two arteries and a single vein. As these vessels grow they form intertwining spirals, embedded in a simple ‘jelly’ with a thin outer covering, so that the mature cord resembles a soft, twisted rope. The fetal heart pumps its own blood via the umbilical arteries to the placenta, where their finest branches lie bathed in the mother's blood; they are drained by the tributaries of the umbilical vein which takes it back in the cord to the fetus, to flow back to the heart. Thus ‘used’ blood is pumped through arteries and ‘refreshed’ blood is returned to the heart by veins (comparable to the flow to and from the lungs after birth).

By full term, the cord is about 50 cm long, looped within the amniotic sac, allowing freedom of movement of the mature fetus. During labour, the cord is occasionally a hazard: it can prolapse and be compressed by the descending head against the mother's pelvis, obstructing the blood flow, or it may encircle the baby's neck and need to be loosened by the midwife. It is of course the custom to cut the cord, between two clamps, as soon as the infant is born. Attention can then be given separately to the infant, and to the mother for delivery of the placenta. But there are also natural mechanisms that promptly reduce the blood flow in the umbilical arteries. The start of breathing alters the mechanics of the heart and circulation such that blood now flows preferentially through the lungs to become oxygenated. Various chemical changes, as well as stretching and cooling of the cord, can contribute to close-down, by constriction of the smooth muscle of the umbilical vessels.

The cord shrivels and separates from the navel within a week or two.

Sheila Jennett

See also antenatal development; belly button; growth and development: birth and infancy; placenta.