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Antenatal hydronephrosis and vesicoureteral reflux (VUR)
Renal pelvic dilation in pregnancy is common and usually transient; postnatal ultrasound is done after 48–72 hours (except when posterior urethral valves are suspected), and VUR is diagnosed by VCUG and managed according to grade, infections and bladder-bowel function.
Why it matters in the exam
The first question is timing: in pregnancy, a pelvic diameter above 4 mm in the second trimester or above 7 mm in the third is abnormal. After birth, ultrasound waits 48–72 hours, because physiological dehydration hides dilation. The exception: a boy with bilateral dilation, a thick-walled bladder and a "keyhole" sign — suspected posterior urethral valves (PUV), which need early ultrasound, a catheter and a VCUG.
The second question is when to do a VCUG. Per AAP 2011, after the first febrile UTI at 2–24 months a renal ultrasound is done; a VCUG only when the ultrasound is abnormal (dilation, scarring) or after a recurrent febrile UTI. VUR is graded I–V; most low grades resolve with growth. The RIVUR trial showed that TMP-SMX prophylaxis roughly halves recurrent infections but does not reduce scarring — so it is selective (high grade, bladder and bowel dysfunction).
The common cause of persistent dilation with obstruction is ureteropelvic junction (UPJ) obstruction, diagnosed by a diuretic MAG3 renogram. Treating constipation and bladder and bowel dysfunction (BBD) reduces infections and VUR. Surgery (endoscopic injection or ureteral reimplantation) is reserved for breakthrough infections, persistent high-grade VUR or new scars. DMSA shows renal scars.
Key points
- Pelvic diameter above 4 mm in the second trimester or 7 mm in the third — abnormal.
- Ultrasound after 48–72 hours, except when PUV is suspected.
- VCUG after the first UTI only when the ultrasound is abnormal.
- Prophylaxis in VUR is selective; it reduces infections, not scars.
- UPJ — MAG3 renogram; scars — DMSA.
Summary table
| Situation | Finding | Test | Approach |
|---|---|---|---|
| Antenatal pelvic dilation | Above 4 mm in the second trimester, above 7 mm in the third | Ultrasound 48–72 hours after birth | Follow-up; most resolve |
| Posterior urethral valves (PUV) | Boy, bilateral dilation, thick bladder, oligohydramnios | Early ultrasound and VCUG | Catheter at once, valve ablation |
| UPJ obstruction | Pelvic dilation without ureteric dilation | Diuretic MAG3 renogram | Pyeloplasty if function declines |
| VUR | Febrile infections, dilation | VCUG, grades I–V | Follow-up; selective prophylaxis; surgery for breakthrough infections or high grade |
| Renal scarring | After pyelonephritis | DMSA | Follow blood pressure and proteinuria |
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Sources
- AAP Clinical Practice Guideline: Urinary Tract Infection — Diagnosis and Management of the Initial UTI in Febrile Infants and Children 2–24 Months, 2011 (reaffirmed 2016) — Imaging after the first febrile UTI
- Multidisciplinary consensus on the classification of prenatal and postnatal urinary tract dilation (UTD system), 2014 — Prenatal thresholds and postnatal follow-up
- RIVUR Trial Investigators, Antimicrobial Prophylaxis for Children with Vesicoureteral Reflux (NEJM), 2014 — Effect of prophylaxis on recurrence and scarring
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.