The kidney (metanephros) arises from the level of the upper sacral segments (S1, S2), which is why they are initially supplied with blood by the sacral branches of the aorta. Between weeks 5 to 8, upward movement (ascent of the kidneys) takes place up to the level of L2 (location of the renal hilum). The hilum is shifted at the same time from a ventral position towards a more medial position. Due to this ascent, more cranially-located branches of the aorta take over the blood supply. Excretion of urine begins in month 3, into the amniotic cavity.