Hydronephrosis (HDN) is defined as dilatation of renal pelvis and calyceal system. Hydronephrosis can be unilateral or bilateral, obstructive or non-obstructive; congenital or acquired. Hydronephrosis in fetus/newborn is increasingly recognised due to widespread use of fetal ultrasound scanning, advanced USG equipment and greater expertise. It is estimated that urinary tract dilatation in utero is identified in 1% all pregnancies but in only 1/5th of these, the abnormality is significant. The best method of diagnosing HDN is by USG of kidneys ureters and urinary bladder, so that the cause of HDN can be detected.

Common causes of hydronephrosis are as follows:

• Pelviureteric junction obstruction (PUJ obstruction)
 
• Vesicoureteric reflux (VUR)
 
• Vesicoureteric junction obstruction
 
• Multicystic kidney
 
• Posterior ureteric valves (PU valves) (can be bilateral)
 
• Obstructive or non obstructive megaureters
 
• Ureterocele
 
• Neurogenic bladder
 
• Prune belly syndrome
 
• Urethral atresia

Antenatal USG evaluation of HDN can detect the above mentioned causes, which need confirmation postnatally by clinical evaluation, micturating cystourethrography (MCU), radionucleide Tc99DTPA or MAG renal scan.

Antenatal USG evaluation includes:

• Serial measurements of renal pelvic AP diameter every 4-6 weekly from 16th week onwards(>10mm significant),
 
• Dilation of calyces, ureters,
 
• Echogenicity of renal parenchyma
 
• Contralateral kidney size, dilation,
 
• Bladder size and thickness,
 
• Emptying time; urine flow,
 
• Amniotic fluid volume( oligohydramnios means poor fetal renal function) and
 
• Posterior urethral dilation.