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gigs

Overview

Produced as part of the Guidance for International Growth Standards project, gigs provides a single, simple interface for working with the WHO Child Growth standards and outputs from the INTERGROWTH-21st project. You will find functions for converting from anthropometric measures (e.g. weight or length) to z-scores and centiles, and the inverse. Also included are functions for classifying newborn and infant growth according to literature-based cut-offs.

gigs is be of use to anyone interested in fetal and child growth, including child health researchers, policymakers, and clinicians. This package is best suited to growth data where the gestational age of each child is known, as the use of the growth standards included in gigs is gestational age-dependent. We recommend you check out the available standards section to see if your anthropometric measurements can be converted to z-scores/centiles by gigs. We recommend using gigs to generate continuous or categorical measures of fetal/newborn/child growth, which can then be used in downstream analyses.

Installation 

# You can install the development version from GitHub with `remotes`:
# install.packages("remotes")
remotes::install_github(repo = "lshtm-gigs/gigs")

Available standards

  • ig_nbs - INTERGROWTH-21st Newborn Size standards (including very preterm)
    Component standards
    Acronym Description Unit gest_days range
    wfga weight-or-gestational age kg 168 to 300 days
    lfga length-for-gestational age cm 168 to 300 days
    hcfga head circumference-for-gestational age cm 168 to 300 days
    wlrfga weight-to-length ratio-for-gestational age kg/cm 168 to 300 days
    ffmfga fat-free mass-for-gestational age kg 266 to 294 days
    bfpfga body fat percentage-for-gestational age % 266 to 294 days
    fmfga fat mass-for-gestational age kg 266 to 294 days
  • ig_png - INTERGROWTH-21st Postnatal Growth of Preterm Infants standards
    Component standards
    Acronym Description Unit x range
    wfa weight-for-age kg 27 to <64 exact weeks
    lfa length-for-age cm 27 to <64 exact weeks
    hcfa head circumference-for-age cm 27 to <64 exact weeks
    wfl weight-for-length kg 35 to 65 cm
  • ig_fet - INTERGROWTH-21st Fetal standards
    Component standards
    Acronym Description Unit x range
    hcfga head circumference-for-gestational age mm 98 to 280 days
    bpdfga biparietal diameter-for-gestational age mm 98 to 280 days
    acfga abdominal circumference-for-gestational age mm 98 to 280 days
    flfga femur length-for-gestational age mm 98 to 280 days
    ofdfga occipito-frontal diameter for-gestational age mm 98 to 280 days
    efwfga estimated fetal weight-for-gestational age g 154 to 280 days
    sfhfga symphisis-fundal height-for-gestational age mm 112 to 294 days
    crlfga crown-rump length-for-gestational age mm 58 to 105 days
    gafcrl gestational age-for-crown-rump length days 15 to 95 mm
    gwgfga gestational weight gain-for-gestational age kg 98 to 280 days
    pifga pulsatility index-for-gestational age 168 to 280 days
    rifga resistance index-for-gestational age 168 to 280 days
    sdrfga systolic/diastolic ratio-for-gestational age 168 to 280 days
    tcdfga transcerebellar diameter-for-gestational age mm 98 to 280 days
    tcdfga gestational age-for-transcerebellar diameter mm 98 to 280 days
    poffga parietal-occipital fissure-for-gestational age mm 105 to 252 days
    sffga Sylvian fissue-for-gestational age mm 105 to 252 days
    avfga anterior horn of the lateral ventricle-for-gestational age mm 105 to 252 days
    pvfga atrium of the posterior horn of the lateral ventricle-for-gestational age mm 105 to 252 days
    cmfga cisterna magna-for-gestational age mm 105 to 252 days
  • who_gs - WHO Child Growth Standards for term infants
    Component standards
    Acronym Description Unit x range
    wfa weight-for-age kg 0 to 1856 days
    bfa BMI-for-age kg/m2 0 to 1856 days
    lhfa length/height-for-age cm 0 to 1856 days
    hcfa head circumference-for-age cm 0 to 1856 days
    wfl weight-for-height kg 45 to 110 cm
    wfh weight-for-length kg 65 to 120 cm
    acfa arm circumference-for-age cm 91 to 1856 days
    ssfa subscapular skinfold-for-age mm 91 to 1856 days
    tsfa triceps skinfold-for-age mm 91 to 1856 days

Conversion functions

Terminology

The package operates with on measured anthropometric values, and can convert between these and z-scores/centiles. Z-scores and centiles represent the location of a measurement within a normal distribution of values, such that:

  • A z-score is the number of standard deviations from the mean for a given anthropometric measurement (e.g. height or weight).
  • A centile represents the proportion of measurements in some distribution which we would expect to be lower than a measurement we’ve taken, represented as a value between 0 and 1.

Usage

Conversion functions are named according to the set of standards in use, the component standard from that set, then the type of conversion. For example, to convert values to z-scores in the weight-for-GA standard from the INTERGROWTH-21st Newborn Size Standards would be: ig_nbs/_wfga/_value2zscore()

Similarly, the conversion of length-for-age values to centiles in term and preterm infants could be performed with the WHO Child Growth standards and INTERGROWTH-21st Postnatal Growth of Preterm Infants standards, respectively:

  • Term infants: who_gs/_lhfa/_value2zscore()
  • Preterm infants: ig_png/_lfa/_value2centile()

If the component standard is not included in the function call, it should be passed to the acronym parameter of the general function call. For example, these two function calls would behave in the same way:

ig_nbs_value2zscore(y = 25.7, gest_days = 182, sex = "F", acronym = "hcfga") |>
  round(digits = 2)
#> [1] 1.18

ig_nbs_hcfga_value2zscore(headcirc_cm = 25.7, gest_days = 182, sex = "F") |>
  round(digits = 2)
#> [1] 1.18

A vector of acronym values can be used if you want to convert with different standards for each input.

Values to z-scores/centiles

These functions allow easy conversion from measured values to z-scores or centiles for the standard used.

# Convert from z-scores for individual values...
ig_nbs_value2zscore(y = 0.785, gest_days = 182, sex = "F", acronym = "wfga") |>
  round(digits = 2)
#> [1] 0

# .. or for multiple inputs
ig_nbs_wfga_value2centile(weight_kg = 0.785,
                          gest_days = seq(175, 196, by = 7),
                          sex = "F") |>
  round(digits = 2)
#> [1] 0.75 0.50 0.25 0.09

# You can do the same for centiles
ig_png_wfa_value2centile(weight_kg = c(2.86, 3.12, 3.12, 3.43, 3.77, 4.10),
                         pma_weeks = 40,
                         sex = "M") |>
  round(digits = 2)
#> [1] 0.10 0.25 0.25 0.50 0.75 0.90

Z-scores/centiles to values

These functions convert z-scores to expected anthropometric measurements. They are mostly useful for the creation of reference curves (see below).

# Convert from z-scores for individual values...
ig_nbs_zscore2value(z = 0, gest_days = 182, sex = "F", acronym = "wfga") |>
  round(digits = 3)
#> [1] 0.785

# .. or for multiple inputs
ig_nbs_wfga_zscore2value(z = 0, gest_days = seq(182, 204, by = 7), sex = "F") |>
  round(digits = 3)
#> [1] 0.785 0.893 1.013 1.147

# You can do the same for centiles
ig_png_wfa_centile2value(p = c(0.1, 0.25, 0.5, 0.75, 0.9),
                            pma_weeks = 40,
                            sex = "M") |>
  round(digits = 2)
#> [1] 2.87 3.12 3.43 3.77 4.11

Reference curves

We can use gigs to generate reference curves for the standards by getting curves for the expected weight at multiple z-scores across multiple gestational ages. We would usually recommend ggplot2 for such visualisation, but do not use it here to reduce our package’s dependencies.

z_score_range <- -2:2
gestage_range <- 168:230
ref <- mapply(z_score_range,
               FUN = function(z) {
                 gigs::ig_nbs_wfga_zscore2value(z = z,
                                                gest_days = gestage_range,
                                                sex = "F")
               })
matplot(ref, x = gestage_range, col = 1:5, type = "l", lty = 2:6,
        xlab = "Gestational age (days)",
        ylab = "Weight (kg)")
title(main = "Weight-for-GA in very preterm newborns")
legend(x = min(gestage_range) + 1, y = ref[length(ref)], legend = 2:-2,
       title = "Z-score", col = 5:1, lty = 2:6)

Classification functions

These functions allow for quick identification of at-risk infants through classification of suboptimal growth. The cut-offs used are sourced from research literature; you can check the function documentation to see these sources.

# Size for gestational age
classify_sfga(
  weight_kg = c(2.1, 3.2, 4.5),
  gest_days = c(252, 259, 290),
  sex = "M"
)
#> [1] SGA AGA LGA
#> Levels: SGA AGA LGA

# Small vulnerable newborns
classify_svn(
  weight_kg = c(2.1, 3.2, 4.5),
  gest_days = c(252, 259, 290),
  sex = "M"
)
#> [1] Preterm SGA Term AGA    Term LGA   
#> Levels: Preterm SGA Preterm AGA Preterm LGA Term SGA Term AGA Term LGA

# Stunting, i.e. low length increase relative to age
classify_stunting(
  lenht_cm = c(42.3, 75.4, 72.83),
  age_days = c(252, 525, 245),
  gest_days = c(238, 259, 266),
  sex = "M"
)
#> [1] stunting_severe stunting        not_stunting   
#> Levels: stunting_severe stunting not_stunting

# Wasting, i.e. low weight increase relative to length/height
classify_wasting(
  weight_kg = c(5.75, 2.18, 5.30, 6.75),
  lenht_cm = c(67.7, 46.6, 55.8, 80.1),
  gest_days = c(268, 247, 250, 278),
  age_days = c(45, 33, 230, 278),
  sex = c("F", "M", "F", "M"),
)
#> [1] wasting_severe wasting        not_wasting    wasting_severe
#> Levels: wasting_severe wasting not_wasting overweight

# Weight-for-age, i.e. low weight increase relative to age
classify_wfa(
  weight_kg = c(2.1, 7.2, 6.1, 9.1, 9.4),
  age_days = c(435, 501, 323, 201, 154),
  gest_days = c(36, 27, 37, 40, 28),
  sex = c("M", "M", "F", "F", "F")
)
#> [1] underweight_severe underweight_severe normal_weight      overweight        
#> [5] <NA>              
#> Levels: underweight_severe underweight normal_weight overweight

Other packages

Other R packages can be used to analyse growth data with international standards, but have limitations which are not present in gigs. There are also software packages external to R which implement these standards. The table below describes these packages, and to what extent they have implemented functions that let users convert anthropometric measurements to z-scores/centiles in each set of standards implemented in gigs - the WHO Child Growth standards, INTERGROWTH-21st Newborn Size standards (including Very Preterm), and the INTERGROWTH-21st Postnatal Growth standards for preterm infants. A green tick (✅) indicates that all possible standards are included in a package, a red cross (❌) indicates that these standards are completely missing, and an exclamation mark (❕) indicates that some of these standards are implemented but not others.

Software Platform WHO (0-5 years) IG-21st NBS IG-21st PNG IG-21st Fetal Functionality
gigs R ✅ ✅ ✅ ✅ Values ↔︎ z-scores/centiles
anthro R ✅ ❌ ❌ ❌ Values → z-scores
childsds R ✅ ❌ ❌ ❌ Values → z-scores/centiles
ki-tools/growthstandards R ✅ ✅ ❌ ❕ Values ↔︎ z-scores/centiles
nutriverse/intergrowth R ❌ ❌ ❌ ❕ NA
gigs Stata ✅ ✅ ✅ ✅ Values ↔︎ z-scores/centiles
zanthro Stata ✅ ❌ ❌ ❌ Values → z-scores/centiles

We have benchmarked some of these implementations against each other for conversion of values to z-scores in the WHO Child Growth standards. At 100,000 inputs, each package took:

Package Time taken (100,000 inputs)
ki-tools/growthstandards 125 ms
childsds 126 ms
gigs 128 ms
gigs 0.41 seconds
zanthro 2.05 seconds
anthro 2.24 seconds
nutriverse/intergrowth NA (no WHO functions)

The results can be seen in the online benchmarking article.

The WHO and INTERGROWTH-21st standards are also available in standalone form, available from the WHO website and INTERGROWTH-21st website, respectively. The INTERGROWTH-21st website also includes download links for Excel-based calculators in some standards.

Citation

Parker S (2023). gigs: Assess Fetal, Newborn, and Child Growth. https://github.com/lshtm-gigs/gigs/.

Code of Conduct

Please note that the gigs project is released with a Contributor Code of Conduct. By contributing to this project you agree to abide by its terms.