Swollen kidney (hydronephrosis) seen by chance on a scan

What the report means

Each kidney makes urine, which drains through a funnel inside the kidney (the renal pelvis and its branches, the calyces) and then down a tube called the ureter to the bladder. (MedlinePlus) Hydronephrosis means that drainage system looks widened. The NHS says it is usually caused by something blocking the flow of pee. (NHS)

Words a report may use (simplified)
Report wording	What it describes
Pelvic dilatation or pyelectasis	Only the funnel (renal pelvis) is wider; one radiology system calls a width of 5 to 7 mm grade I (Rousan 2025)
Mild hydronephrosis	The pelvis and some calyces are wider, with normal kidney tissue (Rousan 2025)(Onen 2020)
Moderate or severe hydronephrosis	Marked widening; higher grades add thinning of the kidney tissue (Rousan 2025)(Onen 2020)
Hydroureter or hydroureteronephrosis	The ureter (the tube to the bladder) looks wide too (Tamir 2025)
Extrarenal pelvis	The funnel sits partly outside the kidney; it can measure wide without much pressure on the kidney (Onen 2020)(Tamir 2025)
Unilateral or bilateral	One side or both sides (Alshoabi 2021)

How grading works

Several systems. Doctors use the front-to-back width of the renal pelvis, the Society for Fetal Urology (SFU) grades, a radiology grade, the urinary tract dilation (UTD) system or the Onen grades. No single method has full agreement. (Onen 2020)(Rousan 2025)
Grades are not exact. A review of grading says SFU grades are subjective and readers agree only moderately, and that pelvis width alone can mislead when the funnel sits outside the kidney. (Onen 2020) Most of that research is in babies and children.
Most findings are mild. In 233 people with hydronephrosis on ultrasound, 58% had grade 2 and 11.6% grade 1, and it was on one side in about 89%. (Alshoabi 2021)

Harmless causes and blockage

A lot to drink and a full bladder. In 56 healthy adults aged 19 to 45, about 47% to 48% showed hydronephrosis 60 to 90 minutes after drinking a large amount of water, mostly on the right side. It was partly better after emptying the bladder, but 12.5% still had a moderate grade at 90 minutes. The authors say this can lead to tests that are not needed. (Rousan 2025)
Pregnancy. Widening is reported in up to 90% of pregnancies, more on the right, and usually causes no symptoms. (Tamir 2025) The NHS lists pregnancy among the causes. (NHS)
Kidney shape. An extrarenal pelvis can make the funnel look wide on a measurement. (Onen 2020)(Tamir 2025)
Blockage. The NHS lists kidney stones, an enlarged prostate, infections, damage to the ureters, a blood clot and some tumours. (NHS) In the 233-person study, ultrasound found a cause in 70.4%, and stones were the cause in 54.1%. (Alshoabi 2021)

These are results from groups of people, not a forecast for one person.

What comes next

Tests the NHS lists. Blood or pee tests to check for infection, an examination to check for an enlarged prostate, and an X-ray, CT or other scan to look for a blockage in the kidneys, bladder or ureters. (NHS)
Kidney function. NICE says to offer an eGFR blood test and a urine albumin test (ACR) to adults with structural renal tract disease, repeated kidney stones or an enlarged prostate. (NICE NG203) See kidney test results for what the numbers mean.
Scan after emptying the bladder. In the hydration study, a scan after passing urine showed less widening. (Rousan 2025) Your doctor decides whether a repeat scan helps.
When kidney function has dropped. For sudden kidney injury with no clear cause or a risk of blockage, NICE says to offer an urgent urinary tract ultrasound within 24 hours, and to refer blockage of the upper urinary tract to a urologist. (NICE NG148)
Earlier scans. Comparing with an older scan helps show whether the widening is new. Bring dates of earlier scans if you have them.

Your doctor decides which tests fit your report, your symptoms and your earlier results.

Who it applies to

Adults told that an ultrasound or CT done for another reason shows a widened kidney drainage system. Hydronephrosis found in an unborn baby on the 20-week scan is a different topic; the NHS says it often gets better on its own. (NHS)

NHS, hydronephrosis: (NHS)

"Ask for an urgent GP appointment or get help from NHS 111 if" you have symptoms such as "pain in the lower back, side or tummy (abdomen)", "difficulty peeing", "blood in your pee" or "a high temperature, or feeling hot, cold or shivery".

The NHS says that if hydronephrosis is not treated or is severe, it can damage the kidneys.

Routine age-based screening

No routine screening test looks for hydronephrosis. It ties in with checks that are part of age-based care:

Kidney tests. See kidney disease screening for who is tested and how often.
Prostate. See enlarged prostate seen by chance.
Bladder. See bladder wall thickening seen by chance.
Other kidney findings. See kidney stone found by chance, small or scarred kidney seen by chance and small kidney mass found by chance.
All screenings by age. See screenings by age and gender.

Where they differ:

How to grade it. Doctors use at least five grading systems, and a review says none has consensus across specialties. (Onen 2020) One report's "mild" may be another's "grade 2".
How to read it. The NHS describes hydronephrosis as usually caused by a blockage that needs treatment soon. (NHS) Imaging studies of healthy adults and pregnant women show widening without a blockage is common, and the hydration study warns against tests that are not needed. (Rousan 2025)(Tamir 2025)
When to scan the kidneys. NICE says not to routinely offer a urinary tract ultrasound for sudden kidney injury when the cause is already known, but to offer one urgently when no cause is found or blockage is a risk. (NICE NG148)

Your doctor decides what your result means for you.

Questions people ask

What is hydronephrosis?

It means the part of the kidney that collects urine is wider than usual. The NHS describes it as a kidney swollen from a build-up of pee, usually because something blocks the flow. Urine normally drains from each kidney down a tube called the ureter to the bladder. (NHS) (MedlinePlus)

Can drinking a lot of water cause it?

Yes, for a short time. In a study of 56 healthy adults, about 47% to 48% showed hydronephrosis on ultrasound 60 to 90 minutes after drinking a large amount of water, mostly on the right side. Emptying the bladder made it partly better. (Rousan 2025)

Is it normal in pregnancy?

Widening of the kidney drainage system is reported in up to 90% of pregnancies and is more common on the right side. It usually causes no symptoms. (Tamir 2025)

What does mild, moderate or grade 1 to 4 mean?

Grades describe how wide the drainage system looks and, at higher grades, whether the kidney tissue looks thinner. Several grading systems exist and none has full agreement, and readers do not always give the same grade. (Onen 2020) (Rousan 2025)

What tests come next?

The NHS lists blood or pee tests, a check for an enlarged prostate, and scans such as CT to look for a blockage. NICE says to offer an eGFR blood test and a urine albumin test (ACR) to adults with structural renal tract disease or an enlarged prostate. (NHS) (NICE NG203)