What is unilateral hydronephrosis?

Retained urine: Results in kidney swelling, most commonly due to obstruction between the kidney pelvis (urine funnel) and ureter which drains urine into bladder (uretero-pelvis junction obstruction), less commonly due to obstruction at bladder end of ureter or bladder reflux, in both cases ureter will will also be swollen. Unilateral means that hydronephrosis is only present on one side.

Hydronephrosis: Refers to fluid inside the kidney. This fluid is urine which is able to remain in the kidney because of a larger size of the renal pelvis. The renal pelvis is the part of the kidney where the urine collects prior to descending into the bladder. The size of the pelvis should not exceed 7 mm on ultrasound. Small increases have no clinical significance and may represent variants of normal.