Hydronephrosis is usually caused by a blockage in the urinary tract or something disrupting the normal workings of the urinary tract.

The urinary tract is made up of the kidneys, the bladder, the ureters (the tubes that run from the kidney to the bladder) and the urethra (the tube that carries urine out of the body).

A blockage or problem in the urinary tract can mean urine is unable to drain from the kidneys or is able to flow the wrong way up into the kidneys.

This can lead to a build-up of urine in the kidneys, causing them to become stretched and swollen.

Causes of hydronephrosis in adults

Some of the main causes of hydronephrosis in adults include:

• kidney stones – small stones that form in the kidneys that can sometimes travel out of a kidney and block the ureters
• benign prostatic hyperplasia – non-cancerous swelling of the prostate gland in men
• pregnancy – during pregnancy the enlarged womb (uterus) can sometimes place pressure on the ureters
• narrowing of the ureters – this can occur as a result of injury to the ureter, infection or surgery
• pelvic organ prolapse – where one or more of the pelvic organs bulge into the vagina
• neurogenic bladder – damage to the nerves that control the bladder
• cancers in or around the urinary tract – such as bladder cancer, kidney cancer, prostate cancer, cervical cancer, ovarian cancer or womb cancer

Less commonly, the urinary tract can become blocked or squashed by a blood clot, endometriosis (where tissue that behaves like the lining of the womb is found outside the womb) or ovarian cysts (fluid-filled sacs in the ovaries).

Causes of hydronephrosis in babies

Sometimes it's not clear why hydronephrosis develops in unborn babies (antenatal hydronephrosis).

It's thought it often may be caused by an increase in the amount of urine your baby produces in the later stages of pregnancy.

Usually, the kidneys themselves are normal and the condition gets better by itself before or within a few months of birth.

In some cases, it can be caused by:

• a blockage or narrowing in the urinary tract – this is sometimes caused by the growth of excess tissue, but often there's no clear cause
• vesicoureteral reflux – where the valve that controls the flow of urine between the bladder and the ureters does not function properly, allowing urine to flow back up to the kidneys

These problems will often get better on their own, although occasionally your baby may need to have surgery to correct them.

It's extremely rare for hydronephrosis in babies and children to be caused by a tumour or kidney stones.

Hydronephrosis is usually diagnosed using an ultrasound scan. Further tests may be needed to find out the cause of the condition.

An ultrasound scan uses sound waves to create a picture of the inside of your kidneys. If your kidneys are swollen, this should show up clearly.

Further tests

You may need a number of additional tests to help determine the cause of hydronephrosis.

These may include:

• blood tests – used to check for infection
• urine tests – used to check for infection as well as traces of blood (this could be caused by a kidney stone)
• intravenous urography – an X-ray of your kidneys that's taken after a special dye has been injected into your bloodstream; the dye highlights the flow of urine through your urinary tract, which can be useful for identifying any blockages
• a CT scan – similar to an X-ray, but uses multiple images and a computer to build up a 3-dimensional picture of the inside of your body

Diagnosing hydronephrosis in babies

Hydronephrosis may be diagnosed in your baby while you're pregnant, usually during a routine pregnancy ultrasound scan at around 20 weeks. This is known as antenatal hydronephrosis.

If your baby is diagnosed with antenatal hydronephrosis, you may need to have extra ultrasound scans during your pregnancy to check your baby is growing normally and their kidneys are not getting too large.

In most cases, the problem will improve before your baby is born or within a few months afterwards.

Your baby may need to have scans after they're born to monitor their condition and see if treatment is necessary.

Read more about treating antenatal hydronephrosis.

Treatment for hydronephrosis depends on what's causing the condition and how severe it is.

In adults, the aims of treatment are to:

• remove the build-up of urine and relieve the pressure on your kidneys
• prevent permanent kidney damage
• treat the underlying cause

Most people with hydronephrosis will have a procedure called catheterisation to drain the urine from their kidneys.

Depending on the underlying cause, medication or surgery may be needed afterwards to correct the problem.

If the condition is severe or causing problems such as a urinary tract infection (UTI), you may to be treated soon after you're diagnosed.

In less severe cases, it may be safe to delay treatment for a short period.

Draining the urine

The first stage in treating hydronephrosis is to drain the urine out of your kidneys.

This will help ease your pain and prevent any further damage to your kidneys.

A thin tube called a catheter may be inserted into your bladder through your urethra (the tube that carries urine out of the body) or directly into your kidney through a small cut in your skin.

Read more about urinary catheterisation.

In a few cases where one of the kidneys has already been severely damaged, it may be better to remove the affected kidney. 

Most people can function normally with just one working kidney, which will not usually have a significant effect on your health or lifestyle.

Treating the underlying cause

Once the pressure on your kidneys has been relieved, the cause of the build-up of urine may need to be treated.

Some possible causes and their treatments are described below:

• kidney stones can be removed during an operation or broken up using sound waves – read more about treating kidney stones
• an enlarged prostate can be treated with medication or surgery to remove some of the prostate – read more about treating prostate enlargement
• narrowing of the ureter (the tube that runs from the kidney to the bladder) can be treated by inserting a hollow plastic tube called a stent, which allows urine to flow through the narrowed section – this can often be done without making cuts in your skin
• cancer that's causing hydronephrosis may be treated using chemotherapy, radiotherapy or surgery to remove the cancerous tissue

If hydronephrosis occurs because you're pregnant, you will not usually need any treatment because the condition will pass within a few weeks of giving birth.

In the meantime, catheters can be regularly used to drain urine from the kidneys.

Painkillers and antibiotics can also be given if you're in pain or develop a UTI.

Treating hydronephrosis in babies

Most babies diagnosed with hydronephrosis before they're born (antenatal hydronephrosis) will not need any treatment because the condition will improve before they're born or within a few months of their birth.

There's usually no risk to you or your child, so labour should not need to be started early.

After the birth, your baby may be examined to check for any obvious problems, such as swollen kidneys, but normally you'll be able to take them home with you.

Your baby may need to return to hospital for some scans during the next few weeks to check there aren't any continuing problems.

These scans may include:

• an ultrasound scan – where sound waves are used to create a picture of your baby's kidneys
• a micturating cystourethrogram (MCUG) – where a thin tube is used to pass a special type of liquid that shows up clearly on X-rays into your baby's bladder while a series of X-rays are taken
• a dimercaptosuccinic acid (DMSA) scan or MAG-3 scan – where your baby is injected with a substance that shows up on a special device called a gamma camera; the camera is then used to take pictures of your child's kidneys

In most children, hydronephrosis will get better as they get older. But until scans show there's no longer a problem, your child may need to take antibiotics to reduce their chances of developing a UTI as the urine inside their kidneys can make them more vulnerable to infection.

If hydronephrosis does not get better by itself, your child may need to keep taking antibiotics. Occasionally, surgery may be recommended to treat the underlying cause of the condition.