In the umbilical system it is the right vein that disappears. The left umbilical vein loses its connection to the left sinus horn and forms an anastomosis with the ductus venosus. Oxygenated blood from the placenta thus reaches the heart directly via a single umbilical vein and the ductus venosus. The cardinal veins go on to form a major portion of the thoraco-abdominal vasculature. During the eight week an anastomosis connects the left and right anterior cardinal veins.
Blood shunts through it from left to right.
The caudal end of the left anterior cardinal vein disappears leaving the anastomosis which will become the left brachiocephalic vein. The right anterior cardinal vein degenerates as well and goes on, along with the common cardinal vein, to form the superior vena cava. In the adult the posterior cardinal veins have all but disappeared. The only remaining derivatives are the common iliac vein and the root of the azygos vein. Two additional pairs of veins form to replace the posterior cardinal veins at a later stage.