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Prof. Dr. Ali Avanoğlu
UPJ Obstruction • Diagnosis and evaluation

Degree of Hydronephrosis: What Do the AP Diameter, SFU, and UTD Classifications Mean?

Phrases on an ultrasound report like "AP diameter 14 mm," "SFU 3," or "UTD P2" describe how pronounced the hydronephrosis (kidney swelling) is. This page explains what each of the three scales measures, how they relate to one another, and why the grade alone does not decide the treatment.

Who this is forFamilies of infants and children whose ultrasound report shows an AP diameter, an SFU grade, or a UTD class
Reading≈6 min
ByProf. Dr. Ali AvanoğluUpdated
This page is part of UPJ obstruction.Read the full treatment overview on the hub page
SFU 0 · none
SFU 1 · pelvis only
SFU 2 · pelvis + a few calyces
SFU 3 · all calyces
SFU 4 · parenchymal thinning
UTD mapping · P1 10–15 mm · P2 ≥15 mm / peripheral calyx · P3 parenchymal thinning
SFU 0–4 side by side: none → pelvis only → pelvis + a few calyces → all calyces → parenchymal thinning; UTD mapping below
01 · Why are there three different scales on an ultrasound report?02 · AP diameter: how many millimeters of kidney swelling matter?03 · SFU grading: 0 to 4, by appearance04 · UTD classification: what P1, P2, P3 mean and recommend05 · Why doesn't the grade alone decide anything?
In brief5 madde
  • Why are there three different scales on an ultrasound report?: Hydronephrosis is widening of the kidney's collecting system with urine; it is a finding, not a diagnosis.
  • AP diameter: how many millimeters of kidney swelling matter?: The AP diameter is the front-to-back diameter of the renal pelvis, measured on ultrasound.
  • SFU grading: 0 to 4, by appearance: The SFU system looks not at millimeters but at the shape of the image: is the widening confined to the pelvis alone, has it spread to the calyces…
  • UTD classification: what P1, P2, P3 mean and recommend: UTD defines separate stages for before birth (A1, A2–3) and after birth (P1–P3).
  • Why doesn't the grade alone decide anything?: A high grade raises the likelihood of obstruction but does not prove it.
01

Why are there three different scales on an ultrasound report?

Hydronephrosis is widening of the kidney's collecting system with urine; it is a finding, not a diagnosis. Ultrasound measures and grades this widening, and radiologists use three different systems to describe it.

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  • The AP diameter is a single number, in millimeters. The SFU (Society for Fetal Urology) grading is an appearance-based scale from 0 to 4. The UTD (Urinary Tract Dilation) classification, developed in 2014 through the joint decision of several specialty societies, is a newer system that combines the AP diameter with other ultrasound findings.
  • According to the EAU/ESPU guideline, none of these systems is inherently superior to the others. Ultrasound findings are interpreted together; whichever one appears in your report, your doctor reads it alongside the other findings.
02

AP diameter: how many millimeters of kidney swelling matter?

The AP diameter is the front-to-back diameter of the renal pelvis, measured on ultrasound. It is measured in millimeters at the widest cross-section of the kidney, while the baby lies on their back.

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  • On a post-birth measurement, under 10 mm is considered normal. 10–15 mm is counted as mild, and 15 mm or more as more pronounced widening. The thresholds before birth are different: 4 mm or more at 16–27 weeks, and 7 mm or more after 28 weeks, is reported as "widening."
  • The measurement can shift by a few millimeters depending on how much fluid the baby has had, whether the bladder is full, and who is measuring. That is why what matters is not a single number but the trend of measurements taken under similar conditions over time. This is also why the first post-birth ultrasound is done at least 48 hours after birth — a newborn's relative lack of fluid in the first days can mask the widening.
03

SFU grading: 0 to 4, by appearance

The SFU system looks not at millimeters but at the shape of the image: is the widening confined to the pelvis alone, has it spread to the calyces (the kidney's inner chambers), and has the kidney tissue thinned?

  • Grade 0: No widening.
  • Grade 1: Only the renal pelvis is mildly widened.
  • Grade 2: The pelvis and a few calyces are widened.
  • Grade 3: The pelvis and all calyces are widened; the kidney tissue (parenchyma) is normal thickness.
  • Grade 4: In addition to Grade 3, the kidney tissue has thinned.
  • SFU 1–2 is called mild, and SFU 3–4 high-grade hydronephrosis. Thinning of the tissue (grade 4) can be a sign that obstruction is starting to affect the kidney tissue itself, which is why it draws extra attention.
04

UTD classification: what P1, P2, P3 mean and recommend

UTD defines separate stages for before birth (A1, A2–3) and after birth (P1–P3). The post-birth classification evaluates the AP diameter together with the state of the calyces, the ureter, the parenchyma, and the bladder.

  • P1 (low risk): AP diameter 10–15 mm and only the central calyces widened.
  • P2 (moderate risk): AP diameter 15 mm or more, or widening of the peripheral calyces, or a wide-looking ureter.
  • P3 (high risk): Additional findings such as parenchymal thinning, increased echogenicity, cortical cysts, or a bladder abnormality.
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  • The UTD consensus also suggests a follow-up framework for each stage. In P1, ultrasound is repeated within 1–6 months; VCUG, prophylactic antibiotics, and MAG3 are left to the doctor's judgment. In P2, ultrasound is repeated at 1–3 months; VCUG and MAG3 are again a matter of clinical judgment. In P3, ultrasound is repeated at around 1 month, VCUG and prophylactic antibiotics are recommended, and functional imaging (MAG3) is frequently done.
  • These recommendations are a framework, not a fixed rule; your child's age, whether they have a solitary kidney, bilateral involvement, and any history of infection all change the schedule.
05

Why doesn't the grade alone decide anything?

A high grade raises the likelihood of obstruction but does not prove it. A very wide pelvis can still empty urine well with no obstruction at all; conversely, a moderate degree of widening can be associated with significant obstruction.

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  • In the literature, the large majority of mild prenatal hydronephrosis (AP <10 mm, SFU 1–2, UTD P1) — roughly 80–98% — has been reported to resolve on its own or stay stable. In moderate and severe grades, the likelihood of finding a post-birth pathology rises notably. Even so, only roughly a quarter of children with SFU 3–4 eventually need pyeloplasty (the repair operation for UPJ obstruction); most are followed without surgery.
  • So the grade is used not to make the decision itself, but to determine how often to follow up and whether MAG3 diuretic renography (a kidney scan) is needed. A high grade (UTD P2–P3, SFU 3–4, AP diameter over 15 mm) usually calls for measuring kidney function and emptying; mild, stable widening is mostly followed with serial ultrasound.
  • If the grade in your report is rising over time, jumping a category, or the kidney tissue is starting to thin, ask your doctor about it. These questions are covered in detail under high-grade hydronephrosis and serial ultrasound follow-up; having the same team compare measurements from diagnosis through follow-up makes it easier to tell a real change from measurement variation.
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Frequently asked questions
Is SFU 3 hydronephrosis dangerous?
SFU 3 is a picture in which the pelvis and all the calyces are widened but the kidney tissue is preserved. On its own it does not mean danger; but it does call for assessing function and emptying with MAG3 and for closer ultrasound follow-up. Most children in this group are followed without surgery; some go on to surgery during follow-up.
What does an AP diameter of 15 mm mean?
After birth, 15 mm is the start of the UTD P2 stage and is classed as "moderate risk." This value alone does not mean surgery; a repeat ultrasound at 1–3 months and a clinical judgment on whether MAG3 is needed are recommended. Keep in mind that the measurement is affected by bladder fullness and fluid intake.
One report says SFU, another says UTD — which one is right?
Both are correct — they are simply different languages. SFU looks at appearance; UTD evaluates the AP diameter together with the ureter, parenchyma, and bladder. Your doctor takes the measurement conditions into account when comparing two reports, and may ask for a repeat ultrasound at the same center if needed.
Does the grade of hydronephrosis go down on its own?
In mild grades, very often yes; improvement or stabilization is common in the first 2–3 years. In high grades, some also improve, but MAG3 and serial ultrasound are needed in this group to confirm that function is being preserved.
Next • Diagnosis and evaluationWhen Is VCUG (Voiding Cystourethrogram) Needed for Suspected UPJ Obstruction?→
Related pagesFull index →
BasicsKidney Swelling in InfantsKidney swelling (hydronephrosis) seen on a post-birth ultrasound is one of the most common urinary findings in infants, and most of it is temporary. This page explains the causes, the millimeter thresholds, the likelihood of it resolving on its own, and how follow-up works.Follow-up or surgery?High-Grade Hydronephrosis (SFU 3–4, UTD P3)An ultrasound report reading SFU 3–4 or UTD P3 shows that the kidney's swelling (hydronephrosis) is at an advanced level. This raises the likelihood of an underlying problem and the chance of needing surgery; but a high grade alone is not an indication. Function, drainage, and the course over time are evaluated together.Conservative follow-upSerial Ultrasound Follow-upThe backbone of follow-up for UPJ obstruction and hydronephrosis (kidney swelling) is serial ultrasound. It involves no radiation, needs no sedation, and is easy to repeat. This page explains how often check-up ultrasounds are done and which measurements the report tracks.BasicsWhat Is Kidney Swelling (Hydronephrosis)Kidney swelling (hydronephrosis) is a finding, not a diagnosis; UPJ obstruction is only one of its possible causes. In most babies whose ultrasound shows widening, the cause is temporary and needs no treatment at all. This page explains how we tell the two apart.
UPJ Obstruction topic clustersMain treatment page →
Basics 7Diagnosis and evaluation 6Follow-up or surgery? 7Conservative follow-up 5Pyeloplasty and approaches 7After surgery and follow-up 6By age, and special situations 7
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