**4.1 Prenatal ultrasound**

In developed countries, antenatal diagnosis of congenital urinary obstructions is often made in mid-gestation (at 18-20 weeks), when many pregnant women undergo a detailed prenatal ultrasound. Megacystis with or without oligohydramnios is the characteristic ultrasound finding of lower urinary tract obstruction, whereas hydronephrosis may signal upper or lower urinary tract obstruction. Hydronephrosis is the most commonly detected anomaly on prenatal ultrasound, found in as many as 4.5% of fetuses (Ismaili, et al., 2003). Prenatal hydronephrosis may result from non-obstructive processes such as primary VUR or physiologic dilatation as well as from obstruction. Differentiating obstruction from nonobstructive causes of congenital hydronephrosis is critical because the prognosis and management vary significantly among these conditions. Factors for consideration in assessing fetal hydronephrosis include gestational age; gender; whether the finding is unilateral or bilateral; the degree of dilatation; the ultrasonic appearance of the renal parenchyma, including presence/absence of a kidney, echogenicity, evidence of cysts or dysplasia, cortical thickness, and corticomedullary differentiation; the presence, volume, and structure of the bladder; any evidence of dilatation elsewhere in the urinary tract; the presence of other abnormalities of the urogenital system (such as duplication) or outside the urinary tract; amniotic fluid volume; and the progression of all findings over serial evaluations (Pates & Dashe, 2006). Several diagnostic algorithms have been proposed for the evaluation of patients with prenatally detected hydronephrosis (de Kort et al., 2008; Ismaili et al., 2005; Karnak et al., 2009; Riccabona et al., 2008; Shokeir & Nijman, 2000; Yiee & Wilcox, 2008).
