Umbilical veins

The v. umbilicalis impar arises outside the embryo in the connecting stalk, from venules that collect blood from the stems of the chorionic villi. At S9, the unpaired vein of the umbilical cord connects to the intra-embryonic right and left umbilical veins at the caudal margin of the umbilicus. These two veins run laterally from the umbilical coelom and rostrally towards the right and left sinus horns, where they end laterally in relation to the omphalomesenteric veins. At S14, they connect to the capillary plexus in the septum transversum, and open into the hepatic sinusoids. Thereafter, the right umbilical vein degenerates. At S13/14, the left umbilical vein disconnects from the left sinus horn.