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-21^st 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-21^st 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-21^st 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-21^st 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-21^st 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-21^st 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-21^st Newborn Size standards (including Very Preterm), and the INTERGROWTH-21^st 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-21^st NBS	IG-21^st PNG	IG-21^st 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-21^st standards are also available in standalone form, available from the WHO website and INTERGROWTH-21^st website, respectively. The INTERGROWTH-21^st website also includes download links for Excel-based calculators in some standards.

Citation

Parker SR, Vesel L, Ohuma EO (2023). gigs: Assess Fetal, Newborn, and Child Growth with International Standards. 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.