- On ultrasound, the renal pelvis refers to the area where the ureter joins the kidney;
- The width from the back to the front (anterio-posterior diameter) of the renal pelvis can be measured:
- (i) Normal range: < 4 mm at 16-20 weeks; < 5 mm after 20 weeks
- (ii) Dilated renal pelvis: 4-7 mm at 16-20 weeks; 5-9 mm after 20 weeks
- (iii) Hydronephrosis: ≥ 8 mm at 16-20 weeks; ≥10 mm after 20 weeks
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- Normal renal pelvis (RP)
- It is a weak marker for Downs syndrome. A detailed fetal morphology scan is indicated;
- For isolated dilated renal pelvis (i.e. no other ultrasound markers for chromosomal abnormalities), antenatal screening with OSCAR or NIPT test is considered adequate. An amniocentesis is usually not advised, unless the couple wants a 100% certainty on the fetal karyotype;
- The prognosis of isolated dilated renal pelvis is good. It might resolve in the subsequent part of the pregnancy or after delivery. However, less commonly, progression to frank hydroneprhosis is possible and hence a follow-up scan by 28-34 weeks is advised;
- If the renal pelvis remains dilated in the subsequent scan, postnatal ultrasound of the kidneys of the baby is advised to exclude pelvi-ureteric junction (PUJ) obstruction or vescicoureteric reflux (VUR)
