During development, the kidneys typically "ascend" from a position in the pelvis to a position high on the posterior abdominal wall. Although the kidneys are bilateral structures, occasionally the inferior poles of the two kidneys fuse. When this happens, the "ascent" of the fused kidneys is arrested by the first midline structure they encounter, the inferior mesenteric artery. The incidence of horseshoe kidney is about 0.25% of the population.