Multiple grading systems exist for hydronephrosis, and the overlap between them is a genuine source of confusion for patients and clinicians alike. The two most widely used are the Society for Fetal Urology (SFU) system, which grades hydronephrosis from 0 to 4 based on kidney appearance on ultrasound, and the newer Urinary Tract Dilation (UTD) classification, introduced in 2014 to standardize terminology across prenatal and postnatal settings. Neither system is perfect, and a significant push over the past decade has been to clarify where they agree, where they diverge, and where both fall short.
How Hydronephrosis Is Detected Before Birth
Fetal hydronephrosis is typically spotted on a routine prenatal ultrasound. The measurement that matters most at this stage is the anteroposterior (AP) diameter of the renal pelvis, essentially how wide the fluid-filled center of the kidney appears. A widely used threshold defines urinary tract dilation as an AP diameter of 4 mm or more between 16 and 27 weeks of gestation, and 7 mm or more at 28 weeks and beyond.1Indian Journal of Nephrology. Revised guidelines on management of antenatal hydronephrosis Those cutoffs help separate genuinely dilated kidneys from normal anatomical variation, since a small amount of fluid in the renal pelvis is common and harmless in a developing fetus.
Within the abnormal range, severity is broken down further based on gestational age. Between 16 and 27 weeks, dilation from 4 to under 7 mm is considered mild, 7 to 10 mm is moderate, and anything above 10 mm is severe. After 28 weeks, the thresholds shift upward: mild is 7 to under 9 mm, moderate is 9 to 15 mm, and severe is above 15 mm.2American Journal of Obstetrics & Gynecology. Fetal renal pelvic dilation These gestational-age-specific ranges acknowledge that the kidney grows over the course of pregnancy, so a measurement that looks worrisome at 20 weeks might be unremarkable at 34 weeks.
The SFU Grading System
The SFU system was developed to give clinicians a shared visual language for describing hydronephrosis on ultrasound. It assigns a grade from 0 to 4 based on how the calices (the small cups that collect urine inside the kidney), the renal pelvis, and the surrounding kidney tissue appear.3Pediatric Radiology. Ultrasound grading of hydronephrosis: introduction to the system used by the Society for Fetal Urology In broad terms:
- Grade 0: No dilation at all.
- Grade 1: The renal pelvis is mildly dilated but the calices look normal.
- Grade 2: The pelvis is dilated and a few calices are beginning to open up.
- Grade 3: Nearly all the calices are dilated, but the surrounding kidney tissue still looks normal in thickness.
- Grade 4: Calices are dilated and the kidney tissue itself is visibly thinned.
The SFU scale has been used for decades and remains the most recognized system worldwide. Its strength is simplicity. Its weakness, as we will see, is that it lumps together cases with very different outcomes under the same grade, especially at the high end.
The UTD Classification System
Recognizing that the SFU system was designed primarily for postnatal imaging and that different specialties were using inconsistent language, a multidisciplinary group introduced the UTD classification in 2014.4PubMed. 2021 update on the urinary tract dilation (UTD) classification system: clarifications, review of the literature, and practical suggestions The UTD system was built to work across both prenatal and postnatal periods. It uses a risk-stratification approach, grouping findings into categories that suggest how aggressively the case should be investigated or followed.
Prenatally, the system uses UTD-A1 (low risk), UTD-A2-3 (increased risk) based on a combination of AP diameter, caliceal dilation, kidney tissue thickness, ureteral appearance, and bladder abnormalities. Postnatally, the categories shift to UTD-P1, UTD-P2, and UTD-P3, again incorporating multiple ultrasound features rather than a single visual impression. The idea is to capture a more complete picture of the urinary tract than the SFU grade alone provides, and to attach management recommendations to each risk tier.
How Reliably Do Doctors Agree on a Grade
A grading system is only useful if different doctors looking at the same image come up with the same answer. This is where the evidence gets messy. Studies comparing the SFU and UTD systems head-to-head have produced mixed results depending on the imaging method, the reviewers’ specialties, and the grade range being assessed.
One study found that the UTD system achieved moderate agreement among readers for both ultrasound and MRI, while the SFU system showed only fair agreement on ultrasound (improving to moderate with MRI).5PubMed. Inter-rater reliability of the APD, SFU and UTD grading systems in fetal sonography and MRI A larger multicenter study of neonatal imaging reported that agreement was moderate for UTD risk scores and worse for SFU grades.6PubMed. Interobserver and Intra-Observer Reliability of the Urinary Tract Dilation Classification System in Neonates: A Multicenter Study But a third study found overall agreement actually slightly higher for SFU than UTD, while noting that both systems performed poorly for intermediate grades, the very cases that matter most clinically.7PubMed. Six of one, half a dozen of the other: A measure of multidisciplinary inter/intra-rater reliability of the society for fetal urology and urinary tract dilation grading systems for hydronephrosis
The middle grades are the problem in both systems. When the kidney looks totally normal or severely damaged, most clinicians agree. When it falls somewhere in between, the subjective interpretation of caliceal dilation or tissue thinning introduces disagreement. This inconsistency at the intermediate range is clinically significant because those are the cases where the management decision, whether to watch and wait or pursue further workup, is least clear.
Where the Highest Grades Fall Short
Both SFU Grade 4 and UTD-P3 are meant to flag the most severe cases, but they sweep together kidneys with very different amounts of remaining function. A kidney showing mild thinning of the tissue and one that has become essentially a fluid-filled shell both receive the same top-tier grade.8PubMed Central. Grading of Hydronephrosis: An Ongoing Challenge That matters because the child with mild thinning might benefit from early surgery to preserve function, while the child with a near-destroyed kidney might need a completely different strategy. The wide umbrella of the highest grade can obscure the urgency, or lack of it, in an individual case.
This limitation motivated the development of the Onen grading system, which subdivides the high-grade range into more clinically actionable categories. The Onen system was designed to separate children who can safely be followed conservatively from those who are likely to need surgical intervention.8PubMed Central. Grading of Hydronephrosis: An Ongoing Challenge It adds a fifth-grade tier and uses a combination of AP diameter and parenchymal thickness to sharpen the distinction. While it has not achieved the widespread adoption of SFU or UTD, it highlights a real gap in the more common systems.
What the Grade Actually Predicts
For parents who have just been told their unborn or newborn child has hydronephrosis, the most pressing question is almost always: will this go away on its own? The answer depends heavily on severity, and the grading systems are reasonably good at sorting mild from worrisome cases even if they struggle with the middle.
A study following children with isolated antenatal hydronephrosis found that about 72% of those with an AP diameter of 20 mm or less experienced complete spontaneous resolution, with a median time of about six months for milder cases and closer to eleven months for those in the 10–20 mm range.9PubMed. Time to resolution of isolated antenatal hydronephrosis with anteroposterior diameter ≤ 20 mm Another study looking at over a hundred infants found that two-thirds of mild or mild-to-moderate cases resolved, while only about a quarter of moderate-to-severe cases did.10PubMed Central. Hydronephrosis in Infants and Children: Natural History and Risk Factors for Persistence in Children Followed by a Medical Service
The relationship between grade and eventual surgery is even starker. In one series, no child with SFU Grade 2 or lower at the first postnatal ultrasound needed surgery. About one in five with SFU Grade 3 eventually required pyeloplasty (the operation to relieve a blockage at the junction between the kidney and ureter). Among SFU Grade 4 cases, roughly two-thirds needed surgery.11Journal of Pediatric Urology. An alternative grading system to refine the criteria for severity of hydronephrosis and optimal treatment guidelines in neonates with primary UPJ-type hydronephrosis These numbers are useful guides, but they also illustrate the imprecision: a third of SFU-4 kidneys were managed without surgery, while a fifth of SFU-3 kidneys needed it. The grade alone does not dictate the plan.
How Doctors Decide When Surgery Is Needed
Surgical repair (most commonly pyeloplasty) is not triggered by reaching a particular grade on an ultrasound. Instead, the decision rests on a combination of factors tracked over repeated imaging and functional tests. Typical indications include worsening hydronephrosis on serial ultrasounds, a drop in the affected kidney’s share of total kidney function by more than about ten percentage points on nuclear scans, or an initial function below roughly 40% in the setting of an obstructive drainage pattern.12PubMed Central. Ultrasound-Based Scoring System for Indication of Pyeloplasty in Patients With UPJO-Like Hydronephrosis Development of symptoms such as infections, pain, or kidney stones also pushes the decision toward surgery.13Urological Science. Surgical indications for unilateral neonatal hydronephrosis in considering ureteropelvic junction obstruction
This is why grading alone has never been enough: it captures the anatomy at a moment in time but says nothing about whether the kidney is draining adequately or losing function. The trajectory matters more than a single snapshot.
Functional Tests That Go Beyond Ultrasound
The most common functional test is a diuretic renal scan, typically using a tracer called MAG-3. The scan measures how much of the body’s total kidney work each kidney is doing (called differential function) and how quickly fluid drains out after a diuretic is given. A kidney contributing less than about 40% of total function, or draining sluggishly, raises concern about obstruction.
These scans have limitations of their own. A reassuring result on a MAG-3 scan does not always mean the kidney is truly unobstructed, particularly in the setting of high-grade hydronephrosis.14PubMed Central. Is a Reassuring MAG-3 Diuretic Renal Scan Really Reassuring? Moreover, a larger hydronephrotic kidney can actually show inflated function on a nuclear scan because the accumulated fluid absorbs the tracer, giving a misleadingly high reading.15Journal of Urology. Supranormal differential renal function is real but may be pathological: assessment by 99mTechnetium mercaptoacetyltriglycine renal scan of congenital unilateral hydronephrosis Clinicians factor these quirks into their interpretation, which is another reason why management decisions rest on multiple data points rather than any single test result.
MR urography has emerged as an alternative that combines anatomical detail with functional information in a single radiation-free exam.16PubMed Central. Quantitative renal magnetic resonance imaging: magnetic resonance urography It can map the kidney’s drainage in three dimensions while measuring function, and it is particularly useful for complex anatomy or when standard ultrasound and nuclear scans give conflicting signals.17PubMed Central. Use of MR Urography in Pediatric Patients Cost and availability keep it from being a first-line tool in most settings, but it is increasingly used as a problem-solving study.
When Hydronephrosis Hints at Something Else
Hydronephrosis is a description of what the kidney looks like, not a disease in itself. Its causes differ sharply between newborns and adults. In children, the most common underlying reason is a congenital narrowing at the junction where the kidney connects to the ureter. In adults, kidney or ureteral stones account for the majority of cases. One study of over 200 patients found that stones caused hydronephrosis in more than half, while vesicoureteral reflux (urine flowing backward from the bladder toward the kidney) accounted for roughly 7% and pelviureteric junction narrowing for about 4%.18PubMed Central. Etiology of Hydronephrosis in adults and children: Ultrasonographic Assessment in 233 patients
Identifying reflux is particularly important in children because it changes the management plan. When prenatal hydronephrosis is found, some centers perform a voiding cystourethrogram to check for reflux. One study found that when ultrasound revealed additional findings beyond simple dilation, such as a dilated ureter, abnormal kidney shape, or a duplicated collecting system, the odds of detecting reflux were about eight times higher than when dilation was the only abnormality.19PubMed. Evaluation of prenatal hydronephrosis: novel criteria for predicting vesicoureteral reflux on ultrasonography This kind of risk stratification helps clinicians avoid unnecessary invasive tests in low-risk babies while targeting workup toward those most likely to benefit.
Antibiotic Prophylaxis and Infection Risk
Whether to put a baby with hydronephrosis on daily preventive antibiotics is one of the more debated questions in pediatric urology. The concern is that stagnant urine in a dilated system gives bacteria a place to thrive, potentially leading to kidney infections that can scar developing tissue.
For low-grade hydronephrosis, the evidence does not support routine antibiotic prophylaxis. For high-grade cases, a systematic review found that prophylaxis reduced infections, with roughly one infection prevented for every seven children treated.20PubMed Central. Continuous antibiotic prophylaxis in the setting of prenatal hydronephrosis and vesicoureteral reflux Several additional risk factors weigh into the decision. A prospective study identified female sex, uncircumcised male status, hydroureteronephrosis (dilation involving the ureter as well as the kidney), and vesicoureteral reflux as the strongest predictors of febrile urinary tract infection. Not being on prophylaxis independently carried about a fivefold higher risk of infection.21PubMed. Risk factors for febrile urinary tract infection in children with prenatal hydronephrosis: a prospective study This is why the decision is individualized rather than grade-driven: two children with the same SFU grade may face very different infection risks depending on the underlying cause and their anatomy.
AI-Assisted Grading
Researchers have begun training deep-learning algorithms to classify hydronephrosis grade from ultrasound images, partly to address the inter-rater reliability problem. One early model classifying images across all five SFU grades got the exact grade right about half the time but placed about 94% of images either correctly or within one grade.22PubMed Central. Using Deep Learning Algorithms to Grade Hydronephrosis Severity: Toward a Clinical Adjunct A more recent model achieved about 82% overall accuracy, with highest performance at the extremes: over 92% for normal kidneys and about 88% for SFU Grade 4, but lower accuracy in the middle grades.23Journal of Pediatric Urology. Automated Society of Fetal Urology (SFU) grading of hydronephrosis on ultrasound imaging using a convolutional neural network
The pattern mirrors the human reliability data: the extremes are easy, the middle is hard. These tools are not replacing radiologists or urologists anytime soon, but they have potential as a screening layer, flagging images that need closer review or providing a second opinion in settings where specialist access is limited.
The Emotional Weight of a Prenatal Diagnosis
Grading systems are built for clinicians, but the people on the receiving end of the numbers are usually expectant parents who may have had no reason to think anything was wrong with their pregnancy until a routine scan turned up a dilated kidney. The psychological toll is real and underappreciated.
In one qualitative study, women described the experience of a suspected fetal abnormality as a crisis, marked by anxiety, difficulty understanding the medical information, and a tendency to suppress their feelings and hope for the best until delivery.24PubMed Central. When fetal hydronephrosis is suspected antenatally–a qualitative study Even minimal hydronephrosis, the kind most likely to resolve on its own, can generate significant worry. A study directly comparing parents of babies with mild hydronephrosis to controls found that the degree of concern was significantly greater in the hydronephrosis group, and the researchers concluded that repeated antenatal scanning for minimal cases may be doing more harm than good by fueling anxiety without changing outcomes.25PubMed. Antenatal minimal hydronephrosis: is its follow-up an unnecessary cause of concern?
Prenatal consultation with a pediatric urologist appears to help substantially. One study found that before a consultation, about 69% of mothers reported moderate or extreme worry about the diagnosis. After meeting with a specialist who could explain the likely course and monitoring plan, that figure dropped to about 31%.26Frontiers in Urology. Reduction in maternal anxiety following prenatal pediatric urology consultation Clarity about what a grade means, what it does not mean, and what happens next is often the single most helpful intervention a family receives, well before any imaging follow-up or surgical decision enters the picture.