Obstructive uropathies and antenatal hydronephrosis
How to follow an infant with renal dilation found on fetal ultrasound step by step, without missing an emergency, especially posterior urethral valves.
Step 1 of 14
Objectives
After this course you will be able to
- Classify antenatal and postnatal urinary tract dilation using the UTD system.
- Choose the right timing for the first postnatal ultrasound.
- Know the indications for VCUG, MAG3/DTPA and antibiotic prophylaxis.
- Recognize suspected posterior urethral valves and know the urgent measures.
- Explain the indications for surgery in UPJ obstruction, megaureter and ureterocele.
Key message: most cases of antenatal hydronephrosis are transient, but all of them must be followed. A male infant with bilateral dilation, a thick-walled bladder and oligohydramnios during pregnancy is an emergency until valves have been ruled out.
Basics
Antenatal hydronephrosis in numbers
- 1–4.5%of pregnancies
- ~50% or moretransient, physiological dilation
- 10–30%UPJ obstruction
- 10–20%vesicoureteral reflux
Other causes: megaureter (5–10%), multicystic dysplastic kidney, duplex collecting system, ureterocele, ectopic ureter, and the rare but most severe cause, posterior urethral valves (1–2%).
There is a direct relationship between the degree of dilation and the likelihood of pathology: as the grade increases, the likelihood of persistent pathology and surgery rises.
Basics
UTD classification: antenatal
| Weeks 16–27 | Week 28 and later | |
|---|---|---|
| Normal | APD < 4 mm | APD < 7 mm |
| UTD A1 (low risk) | APD 4 to <7 mm | APD 7 to <10 mm |
| UTD A2–3 (increased risk) | APD ≥ 7 mm | APD ≥ 10 mm |
APD: anteroposterior diameter of the renal pelvis. Regardless of the measurement, the finding is classified as A2–3 if there is peripheral calyceal dilation, ureteral dilation, parenchymal abnormality, a bladder abnormality or unexplained oligohydramnios.
The hydronephrosis tool on this site calculates the UTD grade by gestational week and findings.
Basics
UTD classification: postnatal
| Grade | Findings (more than 48 hours after birth) |
|---|---|
| Normal | APD < 10 mm, no calyceal dilation |
| UTD P1 (low risk) | APD 10 to <15 mm or central calyceal dilation; other findings normal |
| UTD P2 (intermediate risk) | APD ≥ 15 mm, peripheral calyceal or ureteral dilation |
| UTD P3 (high risk) | Parenchymal thinning or abnormal echogenicity, bladder abnormality |
The UTD system is a unified language that replaces the older SFU grades. Besides the APD value, the report should describe the calyces, ureter, parenchyma and bladder.
Basics
Posterior urethral valves (PUV)
A congenital membrane in the posterior urethra in boys. It occurs in about one in 5,000–8,000 boys and is the main obstructive cause of chronic kidney disease in children.
- Antenatal signs: bilateral hydroureteronephrosis, a large thick-walled bladder, the "keyhole" sign, oligohydramnios.
- Postnatal signs: weak or dribbling stream, straining, palpable bladder, urinary tract infection, sepsis, renal failure.
Because of the risk of renal failure, PUV is a condition that requires lifelong follow-up. Even after successful valve ablation, bladder function ("valve bladder") and renal function must be monitored.
Diagnosis
When should the first postnatal ultrasound be done?
- Unilateral, A1 or A2–3: first ultrasound more than 48 hours after birth, within the first month. In the first two days, physiological dehydration can mask the dilation.
- Severe bilateral dilation, solitary kidney, suspected PUV (boy, thick-walled bladder, oligohydramnios): ultrasound within the first 24–48 hours, in the maternity hospital.
- Even if the first ultrasound is normal, in A2–3 cases a repeat ultrasound is done after 4–6 weeks.
A "normal" ultrasound in the first 48 hours after birth may underestimate dilation because of early physiological oliguria. Follow-up should not be stopped on the basis of such a result.
Diagnosis
Who needs a VCUG?
The voiding cystourethrogram (VCUG) shows vesicoureteral reflux and the urethra (PUV).
- Suspected PUV: urgently, in the first days.
- UTD P3 and most P2 cases, especially with ureteral dilation, bilateral dilation, a duplex system or ureterocele.
- Bladder abnormality.
- An infant with hydronephrosis who has had a febrile urinary tract infection.
In isolated UTD P1, VCUG is not routinely recommended.
Diagnosis
MAG3/DTPA scintigraphy
MAG3/DTPA diuretic renography shows the functional impact of obstruction: the separate function of each kidney (split function) and drainage after a diuretic.
- Timing: usually after 6–8 weeks of age, to allow renal maturation.
- Indication: P2–P3 cases, suspected UPJ obstruction or obstructive megaureter.
- The key parameter is split function: below 40% or a decrease of more than 10% on repeat studies is significant.
- In children, the drainage curve alone is not proof of obstruction; the result is interpreted together with the ultrasound trend.
DMSA scintigraphy shows scarring and dysplasia, not obstruction. It is used to confirm the absence of function in a multicystic kidney.
Diagnosis
Urgent measures when PUV is suspected
- Bladder drainage: a small catheter (usually a 5–8 Fr feeding tube). A Foley balloon can irritate the bladder neck.
- Creatinine, electrolytes, acid-base status. Maternal creatinine affects the infant's value in the first days, so the trend matters.
- Be prepared for post-obstructive diuresis: fluid and electrolyte balance.
- If there is a urinary tract infection, intravenous antibiotics.
- Confirmation of the diagnosis by VCUG, then endoscopic valve ablation once the infant is stable.
The lowest (nadir) creatinine level reached in the first year of life is one of the strongest predictors of long-term renal outcome.
Treatment
Follow-up and antibiotic prophylaxis
| Postnatal grade | Follow-up |
|---|---|
| UTD P1 | Ultrasound after 1–6 months; if unchanged, the interval is extended |
| UTD P2 | Ultrasound after 1–3 months; VCUG and MAG3/DTPA by individual decision |
| UTD P3 | Ultrasound after 1 month; VCUG and functional imaging are recommended |
Antibiotic prophylaxis: not needed for every infant with hydronephrosis. The greatest benefit has been shown in infants with high-grade dilation, ureteral dilation or reflux, especially uncircumcised boys. The decision is made together with a pediatric urologist.
In an infant with hydronephrosis, unexplained fever must be taken seriously until urinary tract infection has been ruled out: a properly collected urinalysis and culture.
Treatment
UPJ obstruction
Most cases of hydronephrosis are followed without loss of renal function and improve over time. Indications for surgery:
- Split function below 40% or a decrease of more than 10% during follow-up.
- Increasing dilation on ultrasound, parenchymal thinning.
- Symptoms: pain (intermittent pain episodes in adolescents), urinary tract infection, stones.
- Severe bilateral dilation or severe dilation in a solitary kidney.
The standard operation is Anderson-Hynes pyeloplasty (open, laparoscopic or robot-assisted). The success rate is above 90–95%. After surgery, follow-up is done with ultrasound and, if needed, MAG3/DTPA.
Treatment
Megaureter, ureterocele, MCDK
| Condition | Approach |
|---|---|
| Primary obstructive megaureter | Most improve on their own. If function declines or infections recur: temporary stent or balloon dilation in infants, then ureteral reimplantation |
| Ureterocele (usually the upper pole of a duplex system) | Endoscopic incision for decompression in infants; later reflux and the likelihood of a second operation are monitored |
| Multicystic dysplastic kidney (MCDK) | The nonfunctioning kidney often involutes. Compensatory growth and reflux of the contralateral kidney are monitored; nephrectomy is rarely needed |
Treatment
After valves: lifelong follow-up
- Within 2–3 months after valve ablation, a control VCUG or cystoscopy, and assessment of flow and post-void residual.
- Renal function, blood pressure, proteinuria, growth: regularly.
- Bladder function: a "valve bladder" may present with high pressure, nocturnal polyuria and incomplete emptying; if needed, urodynamics, anticholinergics, catheterization, overnight drainage.
- Planned transition to adult urology and nephrology in adolescence.
The main task of the pediatrician caring for a child with PUV: monitor blood pressure, growth, infections and bladder symptoms, and stay in contact with the nephrologist and pediatric urologist.
Quiz
Quiz: Obstructive uropathies
Test yourself: choose one answer.
-
At 32 weeks of gestation the APD is 12 mm and other findings are normal. What is the UTD grade?
After week 28, APD ≥ 10 mm is classified as A2–3 (increased risk). Follow-up with two postnatal ultrasounds is needed.
-
In an infant with unilateral A1 dilation, when should the first postnatal ultrasound be done?
In the first 48 hours, physiological oliguria can mask dilation. If there is no urgent indication, the ultrasound is done after 48 hours.
-
Which situation requires ultrasound in the first 24–48 hours after birth and urgent measures?
This picture suggests posterior urethral valves: bladder drainage, creatinine and VCUG are needed urgently.
-
Which finding is an indication for surgery in UPJ obstruction?
Split function below 40% or a significant decrease during follow-up is one of the main indications for surgery.
-
In a newborn with suspected PUV, which option is preferred for bladder drainage?
A small tube passes easily through the urethra; a Foley balloon can irritate the bladder neck and cause spasm.
Course completed
Related articles for parents
Sources
- Nguyen HT, Benson CB, Bromley B, et al. Multidisciplinary consensus on the classification of prenatal and postnatal urinary tract dilation (UTD classification system). J Pediatr Urol. 2014;10(6):982–998.
- EAU Guidelines on Paediatric Urology, 2025: Hydronephrosis due to UPJ obstruction; Megaureter; Posterior urethral valves; Ureterocele and ectopic ureter.
- Nguyen HT, Herndon CDA, Cooper C, et al. The Society for Fetal Urology consensus statement on the evaluation and management of antenatal hydronephrosis. J Pediatr Urol. 2010;6(3):212–231.
This material is intended for medical professionals and is based on current clinical guidelines. Decisions about an individual patient are made by the treating doctor.