If the kidneys fail to ascend, then one or both of them lies in an unusual position (renal ectopia, pelvic kidney). If the kidneys remain close to each other in the true pelvis, their lower poles can join and fuse (horseshoe kidney). Anomalies in rotation during the ascent of the kidney can also occur; in this case, the renal pelvis and the ureter do not rotate from a ventral to a medial position, as happens during normal development.
During early development, the kidneys are supplied by numerous aortic branches, which during normal development are replaced by one renal artery. If some aortic branches persist, the resulting abnormality is known as having supernumerary renal arteries.