There are three pairs considered as the main veins in fetal life:
Cardinal veins: This is the important venous drainage system, which consists of the anterior and posterior cardinal veins; the common cardinal veins are formed by the anterior and posterior veins before entering the sinus horn. In the fourth week, a symmetrical system is formed by cardinal veins. From the fifth to the seventh week, additional veins are formed, such as the sub-cardinal veins which drain the kidney, the sacrocardinal veins which drain the lower extremities and the supracardinal veins which drain the body wall by intercostal veins. The appearance of anastomoses between the right and left characterize the formation of the vena cava. Furthermore, between the anterior cardinal veins on anastomosis develops forming the left brachiocephalic vein. The blood from the left side of the head and the upper left extremity is channeled to the right. The last portion of the left brachiocephalic vein enters the left posterior cardinal vein and the left superior intercostal vein. The right common cardinal vein and the proximal portion of the right anterior cardinal vein form the superior vena cava. During the fourth week of development the primary venous drainage of the head is provided by the anterior cardinal veins which ultimate form the internal jugular veins. A plexus of venous vessels form the external jugular veins in the face: drainage of the face and side of the head to the subclavian veins. Other veins are also formed by anastomosis. For example, the left renal vein is formed by anastomosis between the subcardinal veins, while the left common iliac vein is formed by anastomosis between the sacrocardinal veins. The azygos vein is formed by the right supracardinal vein with a portion of the posterior cardinal vein. The hemiazygos vein, which empties into the azygos vein, is formed when the fourth and seventh intercostal veins enter the left supracardinal vein on the left side.
The umbilical veins: Some of these veins connect to the hepatic sinusoids and pass on each side of the liver. Because the proximal part of both umbilical veins and the remainder of the right umbilical vein disappear, the left vein is the only one carrying blood from the placenta to the liver. There is communication between the left umbilical vein and the right hepatocardiac channel and ductus venosus. After birth, the ligamentum teres hepatis and ligamentum venosum are formed by the left umbilical vein and ductus venosus, respectively.
Vitelline veins: The plexus around the duodenum is formed by vitelline veins which pass through the septum transversum before entering the sinus venosus. The importance of these veins is to carry blood from the yolk sac to the sinus venosus.