Renal ectopia and fusion.  Embryologic Basis.
To determine when horseshoe kidneys and crossed fused ectopic kidneys might develop, the authors studied the Carnegie collection of human embryos.  They found that renal fusion must occur before the kidneys ascend between the umbilical arteries.
D000013:Congenital Abnormalities / Q000196:embryology; D006801:Humans; D007668:Kidney / Q000002:abnormalities / Q000196:embryology; D012186:Retroperitoneal Neoplasms / Q000175:diagnosis*; D013131:Spine / Q000196:embryology; D014469:Umbilical Arteries / Q000196:embryology; D014513:Ureter / Q000196:embryology; D014551:Urinary Tract / Q000196:embryology