The supracardinal veins anastomose with the subcardinal veins by 7 weeks and the subcardinals lose their cephalic connections to the postcardinals. Around the same time, a cross-connection arises between the precardinals which will become the left brachiocephalic vein.
By 8 weeks, regression of portions of the left sided veins leaves the common adult pattern of inferior vena cava derived from portions of all three venous systems. The posterior body wall drains via intercostal and lumbar veins into azygous and hemiazygous systems cranially and into the inferior vena cava caudally.
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