KEY FACTS
Terminology
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Duplicated kidney with separate upper and lower pole collecting systems
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Complete or partial duplication: Most in utero are complete
Imaging
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2 separate ureters drain upper and lower poles
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Upper pole prone to obstruction
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Hydronephrosis and hydroureter
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Ureter inserts into bladder lower and more medial than normal (ectopic insertion)
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Creates ureterocele, which appears as cyst within bladder
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Lower pole prone to reflux
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Variable and progressive hydronephrosis usually less than upper pole
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Bilateral duplication in 10-20%
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Severe obstruction may result in upper pole parenchymal cystic change, lower pole typically spared
Top Differential Diagnoses
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Ureteropelvic junction obstruction: Ureter not dilated, variable hydronephrosis
Clinical Issues
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Upper pole prone to infection and renal damage
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Most need early surgical repair
Scanning Tips
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Look carefully at all dilated renal calyx connections to renal pelvis when hydronephrosis present
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Look for 2 renal arteries when duplicationed
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Coronal color Doppler view best
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If ureterocele in bladder, almost always duplicated kidney
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Isolated simple ureterocele rare
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If renal duplication suspected but no ureterocele seen, wait and watch bladder fill and empty
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Large ureterocele may mimic bladder and be missed
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and ureter 
dilatation because of ureterocele 
that herniates into the bladder lumen. That ureter inserts lower and more medial than normal. Reflux into the lower pole ureter also occurs, but the amount of distention is variable in utero.
