The SFU grading system for postnatal hydronephrosis uses user responses regarding pelvicaliectasis and renal parenchymal atrophy to determine the severity of hydronephrosis, grading it from 0 to 4. The gradations indicate the level of severity, with higher grades reflecting more severe hydronephrosis.

The calculator works as follows: 

- Inputs:
  - "Pelvicaliectasis" with options representing the degree of renal pelvis and calyceal dilation:
    - "No dilatation, intact central renal complex"
    - "Renal pelvis dilation without calyceal dilation, splitting of renal pelvis only"
    - "Renal pelvis dilation with dilation of >= 1 major calyces"
    - "Renal pelvis dilation with dilation of all 3 major calyces and/or minor calyces"
  - "Renal parenchymal atrophy" with options indicating the presence or absence of atrophy:
    - "Absent"
    - "Present"
    
- Outputs: 
  - The result of the SFU grading:
    - "Grade 0: No hydronephrosis" if there is no dilatation and renal parenchymal atrophy is absent.
    - "Grade 1: Mild hydronephrosis" if there is renal pelvis dilation without calyceal dilation and atrophy is absent.
    - "Grade 2: Mild hydronephrosis" if there is renal pelvis dilation with dilation of one or more major calyces and atrophy is absent.
    - "Grade 3: Moderate hydronephrosis" if there is renal pelvis dilation with dilation of all major calyces and/or minor calyces, with atrophy absent.
    - "Grade 4: Severe hydronephrosis" if there is renal pelvis dilation with dilation of all major calyces and minor calyces, with atrophy present.
    - "Stage cannot be fully assessed" for any other combination or invalid input scenarios.

The Society for Fetal Urology (SFU) grading of postnatal hydronephrosis is a clinical calculator used in the field of pediatric urology. This grading system is specifically designed for assessing the severity of hydronephrosis, a condition characterized by the swelling of a kidney due to a build-up of urine, in newborns and infants. The SFU grading system is a valuable tool for clinicians as it helps in the diagnosis, monitoring, and management of postnatal hydronephrosis. It provides a standardized method to grade the severity of hydronephrosis, which can guide treatment decisions and predict prognosis. The grading system ranges from SFU Grade 0 (no hydronephrosis) to SFU Grade 4 (severe hydronephrosis). There are no specific exclusion criteria for its use, but it is primarily applicable to the pediatric population, particularly newborns and infants.