Attached to the uterine lining, the placenta is the site of gas exchange between mother and fetus. The singular umbilical vein carries oxygenated blood from the placenta to the fetus, while two umbilical arteries return deoxygenated blood to the placenta. The three vessels coil around one another within the Wharton's jelly of the umbilical cord and enter the abdomen at the umbilicus.
Inside the fetus, the vein courses alongside the falciform ligament and then to the liver's underside. At the transverse fissure, the vein divides into two vessels, one larger than the other. The larger of the two is joined by the portal vein, and together they enter the right lobe of the liver. The smaller vessel, now called the ductus venosus, diverges away from the liver and joins with the inferior vena cava.
Closure of the umbilical vein usually occurs after the umbilical arteries have closed. This prolongs the communication between the placenta and fetal heart, allowing for a sort of autotransfusion of remaining blood from the placenta to the fetus.
Under extreme pressure, the round ligament may reopen to allow the passage of blood. Such recanalization is common in patients with cirrhosis and portal hypertension. Patients with cirrhosis experience rapid growth of scar tissue in and around the liver, often functionally obstructing nearby vessels. Vessel occlusion increases vascular resistance and therefore leads to hypertension. In portal hypertension, the vessels surrounding the liver are subjected to abnormally high blood pressure—so high, in fact, that the force of the blood pressing against the round ligament is sufficient to recanalize the structure.