During the fifth and sixth weeks of development, the mature kidneys lie in the pelvis with their hila pointed anteriorly. As the pelvis and abdomen grow, the kidneys slowly move upward. By the seventh week, the hilum points medially and the kidneys are located in the abdomen. As the embryo continues to grow in a caudal direction, the kidneys are left behind and eventually come to lie in a retroperitoneal position at the level of L1 by the ninth week of development. In the meantime, the kidneys have completed rotation and the hila now face anteromedially.
If ascent does not proceed normally, the result is an ectopic kidney. These may be in the form of pelvic kidneys.
You will notice that in the drawing above, the blood supply to the kidneys is changing as well. As the kidneys ascend into the abdomen, they receive their blood supply from the branches closest to them. This means that as the kidneys move up, their blood supply comes from higher and higher up the aorta. In normal circumstances, the more caudal renal arteries will simply degenerate as they are no longer needed. If this does not occur, it is possible to have accessory renal arteries.
