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.

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
• 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 or ovarian cysts (fluid-filled sacs in the ovaries).

Sometimes it is 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:

• Transient dilatation – the pregnancy hormones have a relaxing effect on the ureters, this often resolves itself when the baby is born
• 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

Babies that are born with ANH may need to be treated with antibiotics to prevent kidney infections, but some problems will get better on their own.

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