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Prof. Dr. Ali Avanoğlu
Vesicoureteral Reflux (VUR) • Diagnosis and evaluation

Kidney Enlargement Before Birth (Antenatal Hydronephrosis) and VUR

Hearing the words ‘kidney enlargement’ or ‘kidney dilation’ on a pregnancy ultrasound worries families; yet most antenatal hydronephrosis is transient and resolves on its own after birth. Even so, roughly 10–20% of these infants are found to have vesicoureteral reflux (VUR). This page explains what the enlargement means and which tests are done, and when, after birth.

Who this is forFamilies of infants found to have kidney enlargement during pregnancy or in the newborn period
Reading≈7 min
ByProf. Dr. Ali AvanoğluUpdated
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01 · What is antenatal hydronephrosis, and how common is it?02 · What do the SFU and UTD grading systems tell us?03 · When is the first ultrasound after birth?04 · Which infants get a VCUG?05 · If reflux is found: the PREDICT study and antibiotic prophylaxis
In brief5 madde
  • What is antenatal hydronephrosis, and how common is it?: Antenatal hydronephrosis is the finding, on ultrasound, that the collecting system of the unborn baby's kidney (the pelvis and calyces where urine collects) appears…
  • What do the SFU and UTD grading systems tell us?: You will see two grading systems in radiology reports.
  • When is the first ultrasound after birth?: Because the newborn is relatively dehydrated in the first 48 hours, kidney enlargement can appear milder than it actually is during this period; that is why, in mild…
  • Which infants get a VCUG?: Not every infant with antenatal hydronephrosis gets a VCUG (voiding cystourethrogram).
  • If reflux is found: the PREDICT study and antibiotic prophylaxis: Reflux found through antenatal hydronephrosis is a different group from reflux found through a febrile infection: the infant has not yet had an infection, most are…
01

What is antenatal hydronephrosis, and how common is it?

Antenatal hydronephrosis is the finding, on ultrasound, that the collecting system of the unborn baby's kidney (the pelvis and calyces where urine collects) appears larger than normal. It is reported in roughly 1–2% of pregnancies and, with today's detailed prenatal ultrasounds, is being noticed more and more often.

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  • The leading cause is transient (physiological) dilation; as the baby grows, the collecting system matures and the dilation disappears. Persistent causes include narrowing at the kidney outlet (ureteropelvic junction obstruction), vesicoureteral reflux, narrowing or dilation where the ureter enters the bladder (megaureter), posterior urethral valves (PUV) in male infants, and duplex collecting system/ureterocele.
  • Vesicoureteral reflux is the second or third most common cause on this list; it is found in roughly 10–20% of infants with antenatal hydronephrosis. But reflux has one important feature: the degree of dilation on ultrasound does not correlate well with whether reflux is present or how severe it is. An infant with mild dilation may have high-grade reflux, while an infant with marked dilation may have no reflux at all.
02

What do the SFU and UTD grading systems tell us?

You will see two grading systems in radiology reports. The SFU (Society for Fetal Urology) system grades dilation from 0 to 4: grades 1–2 are mild (only the pelvis or a few calyces), grade 3 is moderate (all calyces dilated, kidney tissue preserved), and grade 4 is severe (kidney tissue thinned).

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  • The UTD (Urinary Tract Dilation, 2014) classification is newer and more comprehensive; it evaluates the pelvis's anteroposterior diameter (in millimeters), calyceal dilation, the thickness and appearance of the kidney tissue, and the status of the ureter and bladder together. It defines A1 (low risk) and A2–3 (increased risk) groups for the prenatal period, and P1 (low), P2 (intermediate), and P3 (high risk) groups for the postnatal period.
  • These classifications do not diagnose reflux; they predict how severe the dilation is and how closely the infant needs to be followed and which tests are needed after birth. The most important clues pointing to reflux are a visibly dilated ureter, a duplex collecting system, bladder-wall thickening, and involvement on both sides.
03

When is the first ultrasound after birth?

Because the newborn is relatively dehydrated in the first 48 hours, kidney enlargement can appear milder than it actually is during this period; that is why, in mild and moderate cases, the first ultrasound is usually done between day 3–7 and week 4 after birth. Even if the first ultrasound is normal, if marked dilation was reported prenatally, it is checked once more 4–6 weeks later.

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  • In situations such as severe dilation on both sides, an infant with a single kidney, bladder-wall thickening, suspected posterior urethral valves in a male infant, or reduced amniotic fluid before birth, the ultrasound is not delayed and is done within the first 24–48 hours, with a pediatric urology evaluation starting immediately if needed. The reason for this distinction is that early diagnosis of conditions that obstruct the bladder outlet, such as PUV, is decisive for the kidney.
  • In our practice, the evaluation of these infants is planned by the same two specialists from birth onward; the ultrasound, the VCUG if needed, and subsequent follow-up all proceed along a single roadmap.
04

Which infants get a VCUG?

Not every infant with antenatal hydronephrosis gets a VCUG (voiding cystourethrogram). In mild, one-sided dilation with a normal-appearing ureter and bladder, the chance of reflux is not low, but any reflux found is usually low-grade and does not change treatment; most centers choose ultrasound follow-up for these infants and only bring up VCUG if a febrile infection develops or the dilation worsens.

  • Moderate-to-high-grade dilation (SFU 3–4 / UTD P2–P3) or involvement on both sides
  • A dilated ureter on ultrasound (reflux or megaureter)
  • Duplex collecting system, ureterocele
  • Bladder-wall thickening, suspected posterior urethral valves in a male infant
  • Development of a febrile urinary tract infection during follow-up, or worsening dilation
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  • By contrast, an early VCUG is recommended for moderate-to-high-grade dilation (SFU 3–4, UTD P2–P3), a dilated ureter, a duplex collecting system/ureterocele, bladder-wall thickening, and suspected PUV in a male infant. The goal here is to see not only reflux but also the bladder outlet and urethra. In female infants and during follow-up, some centers offer contrast-enhanced voiding urosonography (ceVUS) as a radiation-free option.
  • Practice varies from center to center; explaining to families which finding requires which test, and why testing is or isn't delayed, prevents both unnecessary worry and unnecessary testing.
05

If reflux is found: the PREDICT study and antibiotic prophylaxis

Reflux found through antenatal hydronephrosis is a different group from reflux found through a febrile infection: the infant has not yet had an infection, most are male, and in high-grade reflux the abnormality in the kidney is often due not to infection but to a congenital developmental defect (dysplasia). In these infants, DMSA (renal scan) is used to document the kidneys' baseline status.

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  • The PREDICT study (NEJM 2023) looked at exactly this group — infants aged 1–5 months with grade III–V reflux who had not yet had an infection: in infants given antibiotic prophylaxis (CAP) for 24 months, the first symptomatic UTI dropped from roughly 36% to 21%; no difference was shown in new kidney scarring or function; resistant organisms increased. For this reason, guidelines consider CAP ‘reasonable to offer’ in infants with high-grade reflux, without making it mandatory. Circumcision in male infants is also discussed as an option that reduces infection risk.
  • For low-grade (I–II) reflux, follow-up without antibiotics is usually sufficient; as the child grows, the ureter's tunnel through the bladder wall lengthens and the reflux mostly resolves on its own. In every case, the one skill the family needs to learn is the same: get a urine culture right away, using the correct method, for any unexplained fever.
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Frequently asked questions
Kidney enlargement was seen during pregnancy — does my baby definitely have reflux?
No. Most antenatal hydronephrosis is transient; reflux is found in roughly 10–20% of these infants. The degree of dilation does not predict reflux; the decision is guided by postnatal ultrasound findings and, if needed, a VCUG.
Should my baby have an ultrasound right after birth?
For mild-to-moderate dilation, the first 48 hours can be misleading because of dehydration, so the first ultrasound is usually done between day 3–7 and week 4. For severe dilation on both sides, suspected PUV, or a single kidney, it is done within the first 24–48 hours.
The ultrasound after birth was normal — is follow-up over?
If marked dilation was reported prenatally, one more follow-up ultrasound is done 4–6 weeks later. If both ultrasounds are normal and there is no infection, follow-up is ended for most infants; however, a normal ultrasound does not completely rule out reflux, so the warning to get a urine culture with any febrile infection still applies.
If reflux is found, should antibiotic prophylaxis be started right away?
For high-grade (III–V) reflux, CAP may reasonably be offered based on the PREDICT study; it reduces the first infection, but no effect on scarring has been shown, and there is a risk of resistant bacteria. For low-grade reflux, follow-up alone is usually sufficient. The decision is made together with the family, taking into account the infant's sex, grade, and kidney status.
Prof. Dr. Ali Avanoğlu's publications on this topic · 1
  1. Tiryaki S, Alkac AY, Serdaroglu E, et al. (2013). Involution of multicystic dysplastic kidney: is it predictable? Journal of pediatric urology. PubMed ↗
←Previous • Diagnosis and evaluationVUR Evaluation After a Febrile Urinary Tract Infection: Who Needs a VCUG?Next • Diagnosis and evaluationKidney–Bladder Ultrasound: What It Shows, and What It Doesn't→
Related pagesFull index →
By age and special situationsVesicoureteral Reflux in Infants (0–1 Years)In infants, vesicoureteral reflux (VUR) most often comes to light through antenatal kidney enlargement or a first febrile urinary tract infection. In this age group, both the risk of infection-related kidney damage and the chance of spontaneous resolution are at their highest; the plan for the first year therefore combines protection with patient waiting.Treatment pathwaysAntibiotic Prophylaxis (CAP) in VUR: Who Needs It, and for How Long?In the surveillance of vesicoureteral reflux (VUR), antibiotic prophylaxis (CAP, continuous antibiotic prophylaxis) is a low-dose, once-daily medication given to protect the kidney from infection. It is not needed by everyone; large studies have increasingly clarified who benefits from it. This page explains the evidence, how it is used, and the duration.Diagnosis and evaluationKidney–Bladder Ultrasound: What It Shows, and What It Doesn'tWhen vesicoureteral reflux (VUR) is suspected, the first test is always a kidney–bladder ultrasound: radiation-free, painless, and repeatable. But ultrasound has one limitation: it does not show reflux itself. This page explains what ultrasound looks at, which findings matter, and what a normal ultrasound means.Diagnosis and evaluationVCUG (Voiding Cystourethrogram): How It's Done, and PreparationThe VCUG (voiding cystourethrogram) is the main method that both shows and grades vesicoureteral reflux (VUR). A thin catheter is used to fill the bladder with contrast fluid; as the bladder fills and the child voids, it is imaged to see whether urine backs up into the kidney. This page explains how the procedure is done, how to prepare, radiation, and the alternatives.
Vesicoureteral Reflux (VUR) topic clustersMain treatment page →
Basics 6Diagnosis and evaluation 6By age and special situations 5Bladder and bowel 5Treatment pathways 10Kidney health and infection 5After treatment and follow-up 6
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