The umbilical cord is formed by day 22 dpc, when the mesenchyme of the connecting stalk (or body stalk, formed on day 18 by condensed mesodermal cells basal to the amniotic vesicle), which inhabits extensions of the allantois and extraembryonic omphalomesenteric duct (an intermediate structure formed from the primary yolk sac and a precursor of the yolk sac vesicle, or secondary yolk sac). Between days 28-40, the amniotic cavity surrounds the connecting stalk and the entire embryo and presses the stalk and its contents into a thin tube-like structure that is covered by the amnioic epithelium, forming final umbilical cord structure. It connects the embryo to the developing placenta.
There are normally two arteries and one vein in the umbilical cord. A second umbilical vein, present at very early gestation, disappears without a trace. In 96% of all placentas, the two arteries are connected in the direct vicinity of the cord’s insertion to the placental surface, known as the Hyrtl anastomosis.
The mean length of the umbilical cord is estimated at 55-66 cm, as determined by large-scale studies.