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Hydronephrosis is a condition where one or both kidneys become stretched and swollen as the result of a build-up of urine inside them.

It can affect people of any age and is sometimes spotted in unborn babies during routine pregnancy ultrasound scans. This is known as antenatal hydronephrosis.

Hydronephrosis does not generally cause any long-term problems if it's diagnosed and treated promptly.

Babies with the condition may not require any treatment at all.

The condition can increase your chances of getting urinary tract infections (UTIs).

In severe cases that are left untreated, the kidneys may become scarred, which could lead to loss of kidney function (kidney failure).

Antenatal hydronephrosis (ANH) is increasingly being found in unborn babies during routine ultrasound scans.

It's estimated to show up on at least 1 in every 100 pregnancies.

As a parent, it can be worrying to learn your baby has a problem with their kidneys. But most cases of hydronephrosis in babies are not serious and should not affect the outcome of your pregnancy. Sometimes, babies born with ANH may need to be treated with antibiotics to prevent kidney infections, but most babies born with ANH will have no lasting problems.

Find out more about diagnosing antenatal hydronephrosis and treating antenatal hydronephrosis.

Antenatal hydronephrosis does not usually cause any symptoms. It is often found during pre-natal scans or in tests that are being done for other reasons.

Symptoms for children and adults can include:

• pain in your back or side – this may be sudden and severe, or may be a dull ache that comes and goes over time; it may get worse after you have drunk a lot of fluid
• symptoms of a UTI, such as needing to pee more often, pain or a burning sensation while peeing, and feeling tired and unwell
• blood in your urine
• urinating less often than you used to or with a weak stream

You should see your doctor or contact 111 if you have any of these symptoms.

They may refer you for an ultrasound scan to assess your kidneys.

Antenatal hydronephrosis in babies does not usually cause symptoms after birth, but you should seek medical advice as soon as possible if your baby develops signs of a possible UTI, such as a high fever without any other obvious cause.

Find out about diagnosing hydronephrosis.

Antenatal hydronephrosis

Antenatal hydronephrosis diagnosed in pregnancy is usually mild. It's thought to be caused by an increase in the amount of urine your baby produces in the later stages of pregnancy.

In more severe cases, it may be caused by a blockage in the flow of urine from the kidneys to the bladder, backflow of urine from the bladder to the kidneys, or a blockage in the flow of urine out of the bladder.

Adult hydronephrosis

In adults, hydronephrosis is commonly caused by:

• kidney stones
• pregnancy (maternal hydronephrosis)
• an enlarged prostate gland in men
• narrowing of the ureters (the tubes that carry urine from the kidneys to the bladder) caused by injury, infection or surgery
• some types of cancer, including kidney cancer, bladder cancer, prostate cancer or ovarian cancer

Find out about the causes of hydronephrosis.

If you have hydronephrosis, your treatment will depend on what's causing the condition and how severe it is. 

Pregnant women and babies with the condition may not require any treatment.

In adults, the first stage of treatment is often to drain the urine out of your kidneys by inserting a tube called a catheter into your bladder or kidneys.

This will help relieve the pressure on your kidneys.

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

The treatments used will depend on why the condition developed.

For example:

• kidney stones may be removed during an operation or broken up using sound waves
• an enlarged prostate can be treated with medication or surgery to remove some of the prostate
• blockages in the ureters can be treated using surgery to insert a small tube called a stent
• cancer may be treated using a combination of chemotherapy, radiotherapy or surgery to remove the cancerous tissue

Find out about how hydronephrosis is treated.