Most fetuses presenting with mild hydronephrosis are not problematic, but some are more problematic or may be combined with other anomalies and need to be analyzed comprehensively. Fetal hydronephrosis can be caused by obstructive lesions of the urinary tract and non-obstructive lesions such as vesicoureteral reflux. The most common causes are obstruction of the pyeloureteral junction, vesicoureteral reflux, obstruction of the vesicoureteral junction, posterior urethral valvulae, urethral atresia, and duplicate renal obstruction. Mild renal pelvic dilatation, which in many cases is not pathologic but rather transient, may be normal or physiologic and most often resolves after birth, and a few cases of severe urinary tract obstruction show only mild renal pelvic dilatation. The risk of chromosomal malformations is about 30% when hydronephrosis is combined with multiple malformations, and about 3% when hydronephrosis alone is present. When hydronephrosis is present in the fetus, it should be consulted in a regular hospital and treated under the guidance of a doctor.