What does ultrasound show?
Ultrasound is the most widely used test in pediatric urology. It uses no radiation, is painless and can be repeated. A kidney and bladder ultrasound shows:
- the size, shape and tissue of the kidneys (parenchymal thickness),
- widening of the collecting system (hydronephrosis): the size of the renal pelvis, the calyces,
- widening of the ureters (megaureter),
- the bladder wall and changes inside the bladder (ureterocele),
- urine left in the bladder after voiding,
- stones, cysts, a duplex kidney.
A testicular and scrotal ultrasound shows testicular volume, blood flow, a hydrocele or a varicocele.
With a febrile urinary tract infection, infants and acutely ill children have a kidney and bladder ultrasound within 24 hours; for a child who is not admitted it can be scheduled later. In a toilet-trained child the scan is done before and after peeing to measure the urine left in the bladder (EAU 2026).
Preparation
- The bladder should be full. Give your child water 30–60 minutes before the scan and, as far as possible, do not let him or her pass urine.
- For babies, breastfeeding or a bottle before the scan is enough.
- Fasting is not needed.
Key measurements in the report
- Anteroposterior diameter of the renal pelvis (APD): the main number showing the degree of hydronephrosis. After birth, an APD below 10 mm with no other changes (in the calyces, kidney tissue, ureter or bladder) can be normal. An APD of 10 mm or more needs a separate UTD assessment. You can check your result for orientation with the hydronephrosis tool.
- Calyces: dilation of the central calyces alone can still be low risk, while dilation of the peripheral calyces is a more serious finding.
- Parenchymal thickness: thinning of the kidney tissue may point to long-standing pressure.
- Ureteral diameter: more than 7 mm is considered widened.
- Residual urine: the urine left in the bladder after voiding shows whether the bladder empties completely.
A single measurement is not a diagnosis on its own. The doctor assesses all the findings together with the child’s age and symptoms.
Note for pediatricians and family doctors
In pediatric urologic ultrasound, the bladder should be assessed both full and empty: wall thickness, retrovesical ureteral diameter and post-void residual volume. The report should state the renal pelvic anteroposterior diameter, the state of the calyces, parenchymal thickness and echogenicity, and renal length (with age percentile). In a newborn, ultrasound done in the first 48 hours may underestimate the degree of dilation.
What happens at the visit?
- Bring all previous ultrasound reports and, if possible, the images; they are important for tracking changes over time.
- The scan starts when the bladder is full; then the child passes urine and the residual urine is measured.
Frequently asked questions
The report says 'pyelectasis'. What is that?
Pyelectasis means widening of the renal pelvis; in other words, it is another name for a mild form of hydronephrosis. It is assessed together with the measurement and the other findings.
Different centers give different measurements. Why?
The measurement can vary by a few millimeters depending on how full the bladder is, the child's position and the doctor. If possible, follow-up scans should be done under the same conditions.
Related topics
Sources
This information is for educational purposes and does not replace medical advice. Decisions about your child’s treatment are made with the doctor after an examination.
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