Development of the portal venous systems begins during the fourth week of gestation and ends during the twelfth week. During the fourth week, the vitelline veins form the inferior and middle intervitelline anastomoses and the superior or subhepatic anastomosis, while the umbilical veins divide into two branches, one ending in the primordial liver parenchyma and the other opening into the sinus venosus, the developing heart. By week six, the umbilical circulation is transferred to the liver; all oxygenated blood from the umbilical vein passes through the fetal liver before reaching the heart. Meanwhile, the inferior and middle vitelline vein anastomoses and segments of the left and right vitelline veins begin to form the origin of the mature hepatic portal vein. By the end of week six, the remaining portion of the subhepatic anastomosis enlarges to form the portal sinus, which connects the umbilical circulation to the portal venous circulation, establishing the intrauterine pattern of the hepatic portal system. The umbilical circulation begins to collapse immediately after birth, starting with the umbilical arteries. The fully closed umbilical arteries induce a reversal of blood flow toward the portal sinus and into the portal system. By this stage, the hepatic portal vein and the portal circulation are fully functional (Collardeau-Frachon and Scoazec 2008).