Umbilical vein

The umbilical vein is the single vein within the umbilical cord that delivers oxygenated blood from the placenta to the fetus. The cord normally contains three vessels — one vein and two umbilical arteries — and their roles reverse the usual naming convention: the vein carries oxygen-rich blood toward the fetal heart, while the paired arteries return deoxygenated blood from the fetus back to the placenta.

After entering the fetal abdomen, umbilical venous blood reaches the liver, where much of it is shunted through the ductus venosus directly into the inferior vena cava, bypassing the hepatic circulation. This preserves the highest-oxygen blood for delivery to the heart, from which streaming through the foramen ovale preferentially routes it to the brain and coronary circulation. Umbilical venous blood is the most oxygenated blood in the fetal circulation.

At birth, the placental circulation stops and the umbilical vessels close. The obliterated umbilical vein becomes the ligamentum teres hepatis (round ligament of the liver), which runs in the free edge of the falciform ligament, and the ductus venosus becomes the ligamentum venosum. Clinically, the umbilical vein remains cannulatable for several days after birth, making it a standard route for emergency vascular access in neonates. In portal hypertension, small paraumbilical veins along the ligamentum teres can recanalize, producing the caput medusae pattern.

Fetal circulation is classic USMLE Step 1 material. High-yield points include which vessel carries oxygenated blood (the umbilical vein), the three fetal shunts and their adult remnants, and the liver anatomy connecting the round ligament to its embryologic origin.