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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.

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

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.