What does it mean if a child has mild dilation of the left renal pelvis with no associated hydronephrosis on an ultrasound? what might cause this, and is it something to be concerned about?

Your child probably has a congenital urinary defect which is called ureteropelvic junction, UPJ, obstruction which can be detected by US before birth & it usually gets better when the child gets older. In his case, the abnormality is mild and there is no hydronephrosis so the left kidney function should be intact. I wonder why kidney US was done at 6 years old & if routine fetal US had been done.