Embryology · Urogenital system
Horseshoe kidney and renal ectopia !
- Malformation
The two kidneys join at the bottom and sit lower than normal; sometimes one kidney never rises and stays in the pelvis.
Updated 2026-11-01
See it in 3D: Kidneys · Ureters · Renal pelvis
What it is
In a horseshoe kidney the lower poles of both kidneys fuse while still in the pelvis, and the bridge between them gets caught under the inferior mesenteric artery during the ascent. It is the most common renal fusion anomaly and is usually found by chance. Renal ectopia is a kidney that does not ascend (pelvic kidney) or that crosses to the other side.
Embryological cause
- Weeks 4–6: the metanephric blastemas of both sides, close together in the pelvis, fuse at their lower poles.
- During the ascent (weeks 6–9) the isthmus meets the inferior mesenteric artery and the kidney stays low.
- Medial rotation is incomplete: the renal pelves face forward and the ureters pass in front of the isthmus.
- Abnormal migration of nephrogenic cells has been proposed; genes such as PAX2, HNF1B, SHH and WT1 are mentioned.
- Ectopia: if the ascent fails, the kidney stays pelvic; in crossed ectopia one kidney rises on the opposite side and usually fuses with the other.
Markers
- PAX2
- HNF1B
- WT1
How it presents
- Many people have no symptoms; it is found on an ultrasound done for another reason.
- Urinary infection, stones and flank pain from urinary stasis.
- Hydronephrosis from ureteropelvic junction obstruction.
- Its low, anterior position makes it more vulnerable to abdominal trauma.
- In a girl with a horseshoe kidney, Turner syndrome is worth considering.
Diagnosis
- Ultrasound: low kidneys whose lower poles point to the midline and join in front of the aorta.
- CT or urography: a U-shaped renal mass with the isthmus in front of the great vessels.
- Renal scintigraphy: assesses the function of each side and obstruction.
Test yourself
Think of the answer, then tap the question to see it.
Which structure stops the ascent of a horseshoe kidney?
The inferior mesenteric artery, under which the isthmus joining the lower poles is caught.
Which chromosomal syndromes are associated with horseshoe kidney?
Turner syndrome and trisomy 18.
What is the most common complication of a horseshoe kidney?
Ureteropelvic junction obstruction, followed by stones and urinary infections from stasis.
What is a pelvic kidney?
A kidney that never ascended and stayed in the pelvis, supplied by low arteries such as the common iliac or the lower aorta.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Kidney: pronephros, mesonephros, metanephros and ascent
- Duplicated ureter, UPJ obstruction and posterior urethral valves
- Meiosis and nondisjunction: where trisomies come from
- Renal agenesis and the Potter sequence
Related conditions
- Turner syndrome
- Kidney stones (urolithiasis)
- Wilms tumour (nephroblastoma)
- Urinary tract infection (acute cystitis)
- Vesicoureteral reflux
Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.
Sources: Horseshoe Kidney - StatPearls - NCBI Bookshelf (StatPearls Publishing (NCBI Bookshelf), 2023); Embryology, Genitourinary - StatPearls - NCBI Bookshelf (StatPearls Publishing (NCBI Bookshelf), 2023).
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Representative model, for educational purposes only.