Development of the umbilical cord

The umbilical cord connects the fetus to the placenta, carrying deoxygenated fetal blood to the placenta where it picks up oxygen and glucose, then returns oxygenated blood to the fetus. After birth, the cord is cut, leaving the navel.

In week two, the blastocyst has an inner embryoblast (forming the epiblast and hypoblast) and an outer trophoblast. Cells from the epiblast differentiate into extra-embryonic mesoderm, forming the chorion and chorionic cavity. The embryoblast is connected to the chorion by the body stock, the first structure of the umbilical cord.

By week three, the embryo folds, shaping part of the yolk sac into a gut tube. The mid-gut remains connected to the yolk sac via the vitelline duct, the second umbilical cord structure. The hindgut develops an outpocketing called the allantois, which grows towards the umbilical cord and drains the bladder, becoming the third structure.

In week four, the amniotic cavity folds around the embryo, bringing the body stock, vitelline duct, and allantois together to form the umbilical cord, emerging through the umbilical ring. The amnion swells with amniotic fluid, covering these structures to form an outer membrane for the cord.

Around week six, fetal intestines temporarily protrude into the umbilical cord in a normal process called physiological umbilical herniation, retracting by the third month. A cross-section of the umbilical cord shows the amnion, vitelline duct, allantois, two umbilical arteries, and one umbilical vein.

After cord formation, the vitelline duct and yolk sac shrink and disappear, though sometimes a Meckel's diverticulum remains. The allantois develops into the bladder and leaves behind the urachus in the fetus, becoming the median umbilical ligament post-birth. The umbilical cord eventually contains only the umbilical vein, two umbilical arteries, and Wharton's jelly.

After birth, the umbilical vein transforms into the liver's round ligament, and the two umbilical arteries become the medial umbilical ligaments. The allantois, through the urachus, becomes the median umbilical ligament.

The umbilical cord is primarily made of the body stock, vitelline duct, and allantois. Physiological umbilical herniation is a normal process. The vitelline duct largely regresses (sometimes leaving a Meckel's diverticulum), the allantois forms the median umbilical ligament, the umbilical vein becomes the round ligament of the liver, and the umbilical arteries become the medial ligaments.