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:
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).
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:
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.