Nephroptosis is a downward displacement of the kidney, dropped kidney, or floating kidney, caused by loss of supporting fat. The kidney moves freesly in the abdomen and even into the pelvis. It may cause a kink in the ureter or compression of the ureter by an aberrant inferior polar artery, resulting in hydronephrosis.

Obstruction of the ureter occurs by renal calculi or kidney stones where the ureter joins the renal pelvis (ureteropelvic junction), where it crosses the pelvic brim, or where it enters the wall of the urinary bladder (ureterovesicular junction). Kidney stones at these narrow points result in hydroureter and hydronephrosis.

Hydronephrosis is a fluid-filled enlargement of the renal pelvis and calyces as a result of obstruction of the ureter. It is due to an obstruction of urine flow by kidney stones in the ureter by abnormal blood vessels, or by the developing fetus at the pelvic brim. It has symptoms of nausea and vomiting, urinary tract infection, fever, dysuria (painful or difficult urination), urinary frequency, and urinary urgency. This condition may be corrected by the pyeloplasty, which is a surgical reconstruction of the renal pelvis and ureter to correct an obstruction at the ureteropelvic junction by removing the obstructed portion of the ureter and then reattaching the healthy ureter to the renal pelvis.

The urinary system can become obstructed anywhere along the tract. Calculi (stones) are the most common cause, but blockage can also come from trauma, strictures, tumors, or cysts, spasms, or kinks of the ureters, or congenital anomalies. If not corrected, an obstruction causes urine to collect behind the blockage and generate pressure that may cause permanent functional and anatomic damage to one or both kidneys. As a result of pressure buildup in a ureter, a distended ureter, or hydroureter, develops. Dilation in the renal pelvis is called hydronephrosis.

Hydronephrosis starts as a dilatation in the latter part of the kidney, but back pressure increases so much that the kidney undergoes structural changes.

Treatment is in utero surgical treatment.