Mild Hydronephrosis on Ultrasound: What Does It Mean and Is It Serious?

Mild hydronephrosis means the area inside a kidney where urine collects looks slightly swollen on a scan, usually because urine is not draining freely to the bladder. It is a sign of an underlying issue rather than a disease of its own. Many mild cases have a simple or temporary cause, but the reason still needs to be identified.

This guide is for adults who have an ultrasound or CT report mentioning mild hydronephrosis, mild pelvicalyceal dilatation or mild swelling of one or both kidneys. It covers common causes and follow-up questions. It cannot tell you the cause or how urgent your situation is.

A kidney that is not draining well can become infected, and infection behind a blockage can make people very unwell. Seek urgent medical help the same day, through your doctor, an urgent care service or a phone advice line, if you have mild hydronephrosis and any of the following:

• Fever, a very high temperature, or feeling hot, cold and shivery, especially with pain in the side, back or lower tummy.
• Sudden or severe pain in the side or back, with nausea or vomiting.
• Passing very little urine, or no urine all day.
• Blood in the urine, or painful, cloudy or foul-smelling urine.
• Any of these symptoms while pregnant.

Call your local emergency number or go to an emergency department if someone is confused, very drowsy, has trouble speaking or seems to be getting rapidly worse. The NHS gives this advice for suspected kidney infection. Do not drive yourself, and take a list of your medicines with you.

The NHS lists kidney stones, pregnancy, an enlarged prostate, urinary infections, injury or surgery affecting the ureters, blood clots and some tumours as causes of the blockage behind hydronephrosis. Cleveland Clinic also lists urine flowing backwards from the bladder (vesicoureteral reflux), narrowing from scarring and difficulty emptying the bladder.

Kidney stones are a common cause in adults. A stone in the ureter, the tube from the kidney to the bladder, can partly block urine flow and cause mild swelling above it. NIDDK describes stone pain as sharp pain in the back, side, lower tummy or groin that can come and go in waves.

Hydronephrosis can affect one kidney or both. When only one side is swollen, the cause is often somewhere along that side's ureter. When both are affected, clinicians often look lower down, at the bladder or its outlet.

Sometimes it can play a part. For kidney ultrasound, the sonographer may ask you to drink water so your bladder is full. A very full bladder pushes back on the urinary tract above it. One radiology study using kidney scans found that some kidneys first read as obstructed were reclassified after the person emptied their bladder.

This does not mean every mild finding is caused by a full bladder. If your report does not say whether the kidneys were also looked at after you passed urine, it is reasonable to ask whether that was done or whether a repeat scan would answer the question.

Pregnancy is a recognised cause. Cleveland Clinic explains that the growing uterus can press on the ureters, and hormonal changes may relax the ureter muscles. MedlinePlus also lists an enlarged uterus in pregnancy as a cause of swelling in one kidney.

Because a stone or infection can look similar, pregnant people with new side pain, fever, vomiting or painful urination should contact their maternity team or doctor promptly. Hydronephrosis found in an unborn baby on a pregnancy scan is a separate topic (antenatal hydronephrosis) and is not covered here.

That depends on the cause, your symptoms and your kidney blood tests. Cleveland Clinic says mild cases sometimes resolve on their own and may be managed with a wait-and-see approach. Urine tests, blood tests such as creatinine and eGFR, and sometimes CT or MRI help clarify what is going on.

When there is a clear blockage, treatment aims to restore urine flow. The NHS describes draining urine with a thin tube first in some adults, then treating the cause, for example antibiotics for infection or a procedure for a stone. Treated promptly, hydronephrosis usually causes no long-term problems, but a blockage left untreated can damage the kidney.

MedlinePlus notes that when only one kidney is affected and the other works normally, kidney failure is rare. The risk is higher if the swollen kidney is the only working one, so tell your clinician if you have a single kidney, a kidney transplant or known kidney disease.

If no follow-up has been mentioned, ask whether a repeat scan is planned and when. A mild finding with no symptoms and normal blood tests may simply be rechecked, while a finding that persists or worsens usually leads to further tests to look for the cause. Keep a copy of each report so the next scan can be compared with the last one.

Bring the full scan report and any recent urine and kidney blood test results.

• Is the swelling in one kidney or both, and did the report suggest a cause such as a stone?
• Was my bladder full during the scan, and were the kidneys checked after I emptied it?
• Are my kidney blood tests and urine test normal?
• Do I need a repeat ultrasound or another scan, and when?
• Who will contact me with results, and who do I call if symptoms start before then?
• Which symptoms mean I should seek urgent or emergency care?

On SameCase, we let patients and verified doctors share de-identified cases. Seeing how others with a similar scan report described their symptoms, tests and follow-up can help you decide which details and questions to bring to your appointment.

Similar cases cannot explain your own scan or replace care. If you develop fever, severe pain or stop passing urine, seek urgent help rather than waiting for replies online.