What is hydronephrosis?

Hydronephrosis is when urine builds up inside one or both kidneys causing them to swell. People of all ages can develop hydronephrosis as well as unborn babies (called antenatal hydronephrosis).

Image of a pair of kidneys. The kidney on the right has hydronephrosis. This is caused by a kidney stone which has caused a blockage and a build-up of urine within in kidney.

Causes of hydronephrosis

In adults, hydronephrosis usually develops because of a blockage that stops the normal flow of urine from the kidneys out of the body. Causes of this blockage can include:

Kidney stones

An enlarged prostate (only in biological males)

Some cancers and tumours

Pregnancy

Narrowing of the ureters (tubes that carry urine from the kidneys to the bladder) due to injury or surgery

Blood clot

Blockages or narrowing in the structures that carry urine which develop before birth, such as pelviureteric junction obstruction (PUJO) or posterior urethral valves (PUV) in males

During pregnancy, hydronephrosis in the pregnant person’s kidneys, called maternal hydronephrosis, commonly develops. This is often due to hormonal changes and an enlarging uterus. The larger uterus can compress the tubes that carry urine from the kidneys to the bladder causing a build-up of urine in the kidney.

Symptoms and signs of hydronephrosis

In children and adults, symptoms can include:

Back, side or tummy pain

Signs of a urinary tract infection (UTI), like needing to urinate more often, pain or a burning when urinating, urine that is cloudy or contains blood, fever and feeling tired or unwell.

Urinating less often

Maternal hydronephrosis in most cases doesn’t cause symptoms and is often found during routine pregnancy scans.

Hydronephrosis diagnosis

An ultrasound scan uses sound waves to take pictures of the kidneys. This can be used to look for hydronephrosis and can sometimes also identify blockages, depending on their type, size and location.

Additional tests

Other tests may be needed to help find the cause of hydronephrosis and to look for signs of infection. These can include:

Blood and/or urine tests: to check for infection, kidney function and signs of certain causes and complications of hydronephrosis.

CT scan, X-ray or other medical images: to look for blockages and other causes of hydronephrosis.

Hydronephrosis treatments

In the case of a blockage in the urine pathway, to protect the kidneys from further swelling, a catheter may be inserted. A catheter is a small tube placed into the bladder either through the urethra (the tube that carries urine from the bladder out of the body) or through the tummy.

However, if the site of the blockage is above the bladder different treatments to drain urine out of the body may be needed. This could be a stent (small tube) to open up the ureter(s), or a nephrostomy (a tube that drains urine from the kidneys into a collecting bag).

Pain relief medications may also be prescribed and antibiotics to help fight infection.

Complications of hydronephrosis

If mild and treated quickly and effectively, hydronephrosis usually doesn’t cause long-term issues.

If hydronephrosis is severe or not treated early, it can harm the kidneys and lead to kidney failure.

References

NHS. Hydronephrosis [online]. Nhs.uk, England; Jul 2025 [Accessed 2 September 2025]. Available from: Hydronephrosis - NHS

Preminger, G. Urinary Tract Obstruction [online]. MSD Manual, Merck & Co, Inc., Rahway, NJ, USA; Jul 2025 [Accessed 2 September 2025]. Available from: Urinary Tract Obstruction - Kidney and Urinary Tract Disorders - MSD Manual Consumer Version

NHS. Urinary tract infections (UTIs) [online]. Nhs.uk, England; Jul 2025 [Accessed 2 September 2025]. Available from: Urinary tract infections (UTIs) - NHS

Tidy C. Urinary tract obstruction [online]. Patient.info, Navigate Health Ltd: London, UK; Mar 2022 [Accessed 2 September 2025]. Available from: Urinary Tract Obstruction (Causes, Symptoms, and Treatment)

Ciciu, E. Paṣatu-Cornea, AM. Petcu, LC. and Tuţă, LA. Early diagnosis and management of maternal ureterohydronephrosis during pregnancy [online]. 2021; 5; 23(1):27. [Accessed 12 December 2025]. Available from: <PMCID: PMC8611492>

Hosny, M. Chan, K. Ibrahim, M. Sharma, V. and Vasdev, N. The Management of Symptomatic Hydronephrosis in Pregnancy [online]. 2024; 11;16(1). [Accessed 12 December 2025]. Available from: <PMCID: PMC10784716>

Reviewed December 2025, by Professor Sarah Howles, Consultant urological surgeon.