Growth - Infant Feeds


  
    
      
        Weight:
        
          
          
        
      
      
        Dates:
        
          
          
        
      
    
    
      
        Switch from Fenton to WHO at:
        
          
          
        
      
      
    
  

  
    Patient info — optional
    
      Sex
      
        
        
      
      Date of birth
      
      ⚠ Date of birth is after one or more weight entries — please verify.
      GA at birth
      
        
          
          weeks
          
          days
        
        Birth classification valid for GA 22–42 wks only
      
      Birth weight
      
        
        g
        Z:
        
        
        
      
    
    
      
      
      
      
    
    
  

  
    Weight measurements
    
      Date
      Weight (g)
      
      Z-score
      
    
    
    
    
      Selected interval
      
        
          g/day
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          g/kg/day
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          ΔZ
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      Select exactly 2 rows from the table below
    
    
      
        
          
          
        
      
      
        
      
      
        
          
          
          
        
        
      
      
        
      
      
      
        Custom curve:
        
        %ile
        
        
        
      

      
        
        
        
      
    
  
  
  
    Growth targets
    

    
    
      Maintain current percentile
      
        Over
        
        days
      
      
        
          
            Target weight
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            g/day needed
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            At PMA
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            Target date
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      Reach a target percentile
      
        Target
        
        %ile   over
        
        days
      
      
        
          
            Target weight
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            g/day needed
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            At PMA
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            Target date
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▸ About this calculator

Overview

This calculator supports serial weight monitoring and growth assessment for preterm and term infants. Enter one or more weight measurements with corresponding dates to track weight gain velocity, z-scores, and percentiles over time.

Growth References

Fenton 2025 Preterm Growth Chart

Z-scores and percentiles for infants born before 37 weeks gestational age are calculated using the Fenton 2025 Preterm Growth Chart LMS parameters. The Fenton chart is designed specifically for preterm infant growth monitoring from 22 weeks to 50 weeks postmenstrual age (PMA).

For more information, visit ucalgary.ca/fenton.

WHO Child Growth Standards

Z-scores and percentiles for infants at or beyond the Fenton-to-WHO transition point are calculated using the WHO 2006 Child Growth Standards weight-for-age tables (0–24 months).

For preterm infants, WHO z-scores use corrected age — the infant’s age adjusted for prematurity (PMA minus 40 weeks). For term infants (born ≥ 37 weeks), WHO z-scores use chronological age from birth.

For more information, visit cdc.gov/growthcharts/who-growth-charts.htm.

Z-Score Calculation

Z-scores are calculated using the LMS method:

Z = [ (X / M)^L − 1 ] / (L × S)

where X is the measured weight, and L, M, and S are the LMS parameters corresponding to the infant’s age and sex.

For z-scores beyond ±3 SD, the WHO-recommended SD2/SD3 correction is applied to ensure accurate extreme percentiles.

Z-scores auto-populate when weight, date, sex, date of birth, and gestational age at birth are all entered. The z-score field remains editable — manual entry overrides auto-calculation.

Fenton-to-WHO Transition

The calculator defaults to Fenton up to 50 weeks PMA, then switches to WHO. A toggle at the top of the calculator allows the transition point to be set to 40 weeks PMA instead.

At the transition point, a small discontinuity in z-score (typically ~0.1–0.2 SD) is expected and reflects methodological differences between the two reference populations.

Birth Classification

Size for Gestational Age

SGA, AGA, and LGA classifications at birth are based on the Fenton 2025 sex-specific 10th and 90th percentile weight-for-gestational-age reference values:

SGA (Small for Gestational Age): birth weight below the 10th percentile
AGA (Appropriate for Gestational Age): birth weight between the 10th and 90th percentiles
LGA (Large for Gestational Age): birth weight above the 90th percentile

These classifications use a size-at-birth reference table (not the LMS z-score parameters) and are valid for gestational ages 22–42 weeks. Sex and gestational age at birth are required.

Birth Weight Classification

Classification	Threshold
Extremely Low Birth Weight (ELBW)	< 1000 g
Very Low Birth Weight (VLBW)	< 1500 g
Low Birth Weight (LBW)	< 2500 g
Macrosomia	> 4000 g

Gestational Age Category

Category	Gestational Age at Birth
Extremely preterm	< 28 weeks
Very preterm	28–31 weeks
Moderate preterm	32–33 weeks
Late preterm	34–36 weeks
Term	37–41 weeks
Post-term	≥ 42 weeks

This calculator uses 37 weeks as the threshold for term.

Weight Gain Velocity

Weight gain velocity is calculated between consecutive weight measurements:

g/day = (W₂ − W₁) / days

g/kg/day = 1000 × ln(W₂ / W₁) / days

The logarithmic formula for g/kg/day accounts for the proportional nature of growth and is recommended over simple division for intervals greater than a few days.

ΔZ (change in z-score) between any two time points reflects whether the infant is growing along, above, or below their z-score trajectory.

Important Notes

This calculator is intended as a clinical support tool for registered dietitians and other healthcare professionals. It does not replace clinical judgment. All growth assessments should be interpreted in the context of the infant’s clinical history, nutritional intake, and overall trajectory.

References

Fenton TR et al. Fenton 2025 Preterm Growth Chart. University of Calgary. ucalgary.ca/fenton. Used under CC BY-NC-ND 4.0.
WHO Multicentre Growth Reference Study Group. WHO Child Growth Standards: Methods and development. Geneva: World Health Organization, 2006.
WHO. The WHO Child Growth Standards. who.int/childgrowth/standards.
Cole TJ, Green PJ. Smoothing reference centile curves: the LMS method and penalized likelihood. Statistics in Medicine 1992;11:1305–1319.
WHO. WHO Child Growth Standards: Training Course on Child Growth Assessment. Geneva: WHO, 2008. (SD2/SD3 correction methodology.)