[Timing of ultrasound examination in the detection of neonatal hydronephrosis].

The diagnosis of fetal hydronephrosis requires that there be ultrasound follow-up during the neonatal period. In most cases the dilatation of the renal collecting system regresses and no further diagnostic work-up is needed. A persistently dilated collecting system might be the result of obstruction, in which case surgical intervention should be considered. The ultrasound examination is the key factor in deciding whether to proceed with invasive procedures (cystography, intravenous pyelography or renal isotopic scan) or to stop urological investigation. The case presented emphasizes the importance of the timing of the first ultrasonographic study after birth. Ultrasound examination performed within the first 48 hours can lead to false negative results.