Ureteropelvic junction (UPJ) obstruction is a blockage at the point where part of the kidney attaches to one of the tubes to the bladder (ureters). This blocks the flow of urine out of the kidney. UPJ obstruction mostly occurs in children. It often happens when a baby is still growing in the womb. This is called a congenital condition (present from birth). The blockage is caused when there is a narrowing of area between the ureter and the part of the kidney called the renal pelvis.