The umbilical vein is a vein present during fetal development that carries oxygenated blood from the placenta to the growing fetus.
The blood pressure inside the umbilical vein is approximately 18 mmHg.
Within a week of birth, the infant's umbilical vein is completely obliterated and is replaced by a fibrous cord called the round ligament of the liver (also called ligamentum teres hepatis). It extends from the umbilicus to the transverse fissure, where it joins with the falciform ligament of the liver to separate the left and right lobes of the liver.
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 may be evident 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. This leads to a condition called Caput medusae.
A newborn baby has a patent umbilical vein for at least a week after birth. This umbilical vein may be catheterised for ready intravenous access. It may be used as a site for regular transfusion in cases of erythroblastosis or hemolytic disease.
Additional images 
Model of human embryo 1.3 mm. long.
Scheme of placental circulation.
Diagram of the vascular channels in a human embryo of the second week.
Human embryo with heart and anterior body-wall removed to show the sinus venosus and its tributaries.
Schematic figure of the bursa omentalis, etc. Human embryo of eight weeks.
Liver with the septum transversum. Human embryo 3 mm. long.
Tail end of human embryo twenty-five to twenty-nine days old.
Human embryo, 38 mm, 8-9 weeks.
- Gray's s139 - "Peculiarities in the vascular system of the fetus"
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