Prevalence:

• 1 in 500 births.

Ultrasound diagnosis:

• Dilatation of the collecting system of the kidney observed in the standard transverse view of the abdomen. Ureters and bladder are normal.
• On the basis of the anteroposterior diameter of the pelvis the condition is divided into:
  • Mild (only renal pelvis): 4-7 mm in the 2^nd trimester; 7-9 mm in the 3^rd.
  • Moderate (pelvis and calyces): 8-10 mm in the 2^nd trimester; 10-15 mm in the 3^rd.
  • Severe (cortical thinning): >10 mm in the 2^nd trimester; >15 mm in the 3^rd.

Associated abnormalities:

• Chromosomal defects: low risk in isolated forms.
• Abnormalities of the contralateral kidney: multicystic kidney, ectopia, renal agenesis.
• Associated syndromes are found in 5% of cases. 

Investigations:

• Detailed ultrasound examination (especially of the contralateral kidney).
• Karyotyping should be offered only if other markers are present.

Follow up:

• Ultrasound scans every 4 weeks to monitor the evolution of hydronephrosis and assess amniotic fluid volume.

Delivery:

• Standard obstetric care and delivery.

Prognosis:

• In the majority of cases, the condition remains stable or resolves in the neonatal period. In about 20% of cases, there may be an underlying ureteropelvic junction obstruction or vesicoureteric reflux that requires postnatal follow-up and possible surgery. Moderate hydronephrosis is usually progressive and in more than 50% of cases surgery is necessary during the first 2 years of life.

Recurrence:

• Isolated: no increased risk of recurrence.