Hydronephrosis (hi-dro-nih-FRO-sis) is a swelling of one or both kidneys. It happens when urine (pee) can't drain normally and builds up in the kidney(s). Different things can cause it.

Hydronephrosis doesn't always cause symptoms. When it does, a child might have belly and back pain, blood in the pee, and a higher risk for urinary tract infections (UTIs).

Hydronephrosis is treated by a urologist (urinary tract specialist). Treatment may include just watching to see how your child does, medicines, or sometimes surgery.

Follow your health care provider's instructions for:

• Giving your child any medicines
• Any medicines your child needs to avoid
• Whether it's OK to give acetaminophen (Tylenol^® or a store brand) or any other medicines for pain, if needed
• Taking your child for any imaging studies (such as an ultrasound of the kidney) or any lab tests
• Taking your child to see a urologist
• When to follow up

Your child:

• Has signs of a urinary tract infection (UTI), such as:
  • Fever
  • Vomiting 
  • Bloody or foul-smelling pee
  • Pain when peeing
  • Belly or back pain
  • Fussiness or decreased feeding (in babies)

Your child:

• Has signs of a kidney infection, such as a high fever with shaking or chills or severe belly or back pain
• Is peeing much less than normal or not at all

What causes hydronephrosis? In kids, it's usually from one of these:

• A blockage where a ureter (a thin tube that carries pee from the bladder) meets the kidney or bladder
• A blockage of the urethra (the tube that lets the pee leave the body)
• A backflow (reflux) of pee into the kidneys

What problems can happen in hydronephrosis? Sometimes hydronephrosis can lead to kidney damage, but this doesn't usually happen.