The umbilical cord attaches the developing fetus to the placenta in the uterus. The fetal heart pumps blood through the cord's two to the placenta where it picks up arteriesoxygen and nutrients. The blood then returns through the vein. :Like the pulmonary vein after birth, the arteries contain de-oxygenated blood and the vein contains oxygenated blood. It is primarily made of Wharton's jelly, which is also found in the vitreous humor.
During the first five weeks of development, the embryo embedded in the uterus relies on its own food source and can draw it's meagre oxygen needs directly from the placenta. The umbilical cord develops by the fifth week and continues to provide nutrients and oxygen until birth.
The navel is the attachment point for the umbilical cord, and blood flows in and out of the abdomen during fetal development. After the vein in the cord enters the body, it splits in two with one branch emptying into the hepatic vein and the other into the inferior vena cava. The arteries both branch from the internal iliac artery in the pelvis.
After birth, the temperature change causes the Wharton's jelly to collapse, closing off the arteries and veins within it. This changes the path of blood circulation in the newborn to that of an adult. Left to its own devices, the umbilical cord will fall off on its own within 7-10 days with no complications, but in normal practice it is clamped, cut and tied. Some blood will be lost if the cord is not clamped immediately, but this loss is minimal and of no consequence to the newborn.
It was once believed that a newborn in respiratory distress should remain attached to the cord, believing blood would still flow through the cord to provide some oxygen. However, studies have shown this has no benefit and current practice is to clamp the cord immediately and to start artificial respiration.