Chapter 35. The Umbilical Cord - Review of Medical Embryology Book

A ventral one, the yolk sac stalk, containing the vitelline duct and the vitelline vessels
A caudal one, the connecting stalk, containing the allantois and the umbilical vessels
The connecting stalk progresses ventrally as a result of the cephalocaudal flexion of the embryo and fuses with the yolk sac stalk to form the umbilical cord

THE UMBILICAL CORD brings together, in the same mesenchymal core, the components of the connecting stalk (the allantois and umbilical vessels) and the vitelline duct and vessels
THE UMBILICAL CORD is covered by amnion which is continuous with the outer epithelial layer of the embryo at the attachment of the umbilicus
THE UMBILICAL CORD is short and thick in the young embryo and is inserted in the lower portion of the ventral region of the embryo

With the development of the anterior abdominal wall, the region of umbilical implantation contracts, the cord elongates, and also becomes slender
At term, the cord contains only the umbilical vessels surrounded by Wharton's jelly (a smooth mesenchymous material, a mucous differentiation product of mesoderm) and is about 50-60 cm long and about 2 cm in diameter

It is tortuous, which may result in the so-called false-knots (not significant)
An extremely long cord may encircle the neck of the fetus, creating some problems at the time of delivery; or a very short cord may cause difficulties during delivery by pulling the placenta from its attachment
In about 1% of cases there are so-called true knots in the cord, which in most cases form during labor as a result of the fetus passing through a loop. Since the knots are usually loose, they have no clinical significanc

If the true knot forms early in pregnancy and tightens during active fetal movements, it may interfere with the fetal circulation and cause death and abortion of the embryo or fetus

In one-fifth of all deliveries it loops once around the neck and does not create any fetal risk

In about 1/200 newborns, only one umbilical artery is present and may be associated with cardiovascular abnormalities in 15-20% of cases