Obstruction at ureteropelvic junction (UPJ) leads to renal pelvis and calyceal distention

Renal pelvis dilated, elongated, and bullet-shaped

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  Can extend into pelvis and touch bladder

  
  
  
  
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    Can mimic dilated ureter

  
  
  
  
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  Normal anterior-posterior (AP) renal pelvis diameter

  
  
  
  
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    16 weeks to 27 weeks, 6 days: AP diameter < 4 mm

    
    
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    28 weeks to term: AP diameter < 7 mm

  
  

Dilated calyces can mimic parenchymal cysts

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  Look for connection to renal pelvis

Postobstructive renal cystic dysplasia if severe

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  ↑ renal echogenicity ± renal cysts

Bladder and amniotic fluid normal if unilateral process

Associations often determine prognosis

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  Contralateral renal abnormality in 25%

  
  
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  Bilateral UPJ obstruction in 10%

  
  
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  Extrarenal anomalies in 10%

UPJ may progress rapidly; even mild renal pelvis AP diameter distention (4-7 mm) needs follow up at 32 weeks

Most common significant cause of hydronephrosis

Variable outcomes

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  Many resolve spontaneously and need no treatment

  
  
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  May need surgery (pyeloplasty)

  
  
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  ↑ risk of renal impairment if prenatal AP diameter ≥ 10 mm

Prognosis excellent if unilateral

Use cine sweeps to show all calyces connect to renal pelvis

Massively dilated renal pelvis may cross midline

Beware that normal renal parenchymal pyramids can mimic dilated calyces

Scan complete length of kidney, rule out duplicated kidney with only upper pole obstruction