Having a stone block the ureter is like a tiny rock with sharp edges getting stuck in it. The kidney is still filtering and putting out urine, but it all gets backed up behind that stone, and the battle begins. With time, most stones are eventually pushed all the way through the ureter into the bladder where they pass with the outgoing urine into the outside world. It is very infrequent for a patient to have pain in the middle of the abdomen. The trick is to find out what is going on as quickly, as safely, and as inexpensively as possible.
Early diagnosis is important for two reasons.
First, once diagnosed, the patient can be treated promptly, often with pain medication and a lot of liquids that will continue to build pressure and force the stone along the ureter into the bladder. Unfortunately, there are often all sorts of other stones located in the lower abdomen known as phleboliths, or stones that form in the small venous structures in and around the pelvis. These stones don't hurt or cause any particular damage. They just sit there, quietly disguising the real culprits. So, although helpful, the KUB is usually not diagnostic.