ΓΑΣΤΡΟ /ΝΕΦΡΟΛΟΓΙΚΟ

No agreed international classification exists.
The Australian Society for Ultrasound in Medicine defines hydronephrosis according to gestation by antero-posterior renal pelvic diameter.

18 - 20 weeks	ﾳ 4mm
32 weeks	ﾳ 6mm
any gestation	ﾳ 10mm (regarded as severe hydronephrosis)

80% of fetal hydronephrosis is mild with 20% classified as moderate/severe.

The Society for Fetal Urology considers the appearance of the calyces, renal pelvis and parenchyma to grade the degree of hydronephrosis from I - V (minimal to severe).

	Calyceal dilatation	Size of Pelvis
Grade 1	Physiological	1 cm
Grade 2	Normal calyces	1 - 1.5 cm
Grade 3	Slight dilatation	> 1.5 cm
Grade 4	Moderate dilatation	> 1.5 cm
Grade 5	Severe dilatation, atrophic cortex	> 1.5 cm

Mild fetal hydronephrosis appears to be associated with an excellent prognosis, most likely representing the spectrum of physiological renal pelvic dilatation.

Persistant moderate or progressive fetal hydronephrosis may be due to

Pelviureteric junction obstruction (PUJ)
Vesico-ureteric reflux (VUR).

Severe bilateral hydronephrosis is associated with structural or chromosomal anomaly in 55% of cases.

Posterior urethral valves in the male fetus- may be suggested antenatally by observation of bladder trabeculation post voiding
Multicystic, dysplastic kidney may appear as marked hydronephrosis although echogenic foci can usually be seen within the renal margins
Renal duplications
Megaureters

Table 1

Definition of the grades of antenatal hydronephrosis

ANH grade	APD second trimester, mm	APD third trimester, mm	% with postnatal pathology
Mild	≤ 7	≤ 9	12
Moderate	7–10	9–15	45
Severe	> 10	> 15	88
All	—	—	36

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ANH = antenatal hydronephrosis; APD = anterior–posterior diameter.

Note: The antenatal follow-up of ANH is determined by the severity of the diagnosis. For example, patients with renal or bladder ultrasound signs suggestive of posterior urethral valves (males with bilateral ANH, dilated ureters or dilated thick-walled bladder) require formal prenatal follow-up by fetal–maternal medicine.¹