gigs

Overview

Produced as part of the Guidance for International Growth Standards
project at the London School of Hygiene & Tropical Medicine, 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 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 (GA) of
each child is known, as the use of the growth standards included in
gigs is GA-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 of gigs from
GitHub with pak, or from the
ropensci R-universe:

# install.packages("pak")
pak::pkg_install(pkg = "ropensci/gigs")

install.packages("gigs", repos = "https://ropensci.r-universe.dev")

Rationale + terminology

When working with growth measurements from individual children, it is
helpful to compare those measurements to a growth standard, which
represents average growth for an population of children. This allows
assessment of individual growth - for example, that a baby was born
small, but later caught up to its peers in size. It also allows you to
compare measurements from different children.

These standards of growth can be made in different ways, but the best
studies utilise international samples made of up thousands of healthy
fetuses, newborns, or children. In gigs, you’ll find different
international growth standards, allowing you to compare growth measures
from children with different ages, weights, heights, and more.

In general, you’ll use gigs to transform raw growth measures to
z-scores or 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. In gigs, these are represented as a value between 0
  and 1. For example, 0.5 corresponds to the 50^th centile
  (i.e. the mean), whereas 0.75 corresponds to the 75^th
  centile.

In growth data, z-scores and centiles represent the size a fetus,
newborn, or child relative to its peers. Size here is considered
relative to a standardising variable, which is usually age but could
also be another variable such as their length. By tracking a child’s
relative size as they grow, you can see if they are achieving their
growth potential or not. If not, this may indicate underlying issues
such as ill health or undernutrition.

Classification functions

gigs includes a number of functions which permit fast 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.

Growth classification in data.frame-like objects

Use the classify_growth() function to quickly compute growth
indicators in data.frame-like objects. All classify_*()-style
functions in gigs use
data-masking,
so you provide a data.frame-like object in the .data argument and
then refer to your column names directly. In classify_growth(), you
can also use the .analyses argument to specify which growth indicators
you want to classify.

life6mo_newborns <- gigs::life6mo[life6mo$age_days == 0, ]

# Use classify_growth() to get multiple growth indicators at once
life6mo_classified <- classify_growth(
  .data = life6mo_newborns,
  gest_days = gestage,
  age_days = age_days,
  sex = as.character(sex),
  weight_kg = wt_kg,
  lenht_cm = len_cm,
  id = as.factor(id),
  .outcomes = c("svn", "stunting")
)
#> ! Unused factor levels kept after small vulnerable newborn categorisation:
#> "Term AGA" and "Term LGA".
#> ! Unused factor levels kept after stunting categorisation: "outlier".
#> 
#> ── `gigs::classify_growth()` ───────────────────────────────────────────────────
#> ✔ Small vulnerable newborns: Success
#> ✔ Stunting: Success

head(life6mo_classified, n = 4)
#>    id gestage sex visitweek pma age_days wt_kg   len_cm headcirc_cm  muac_cm
#> 1   1     273   M         0 273        0  2.30 42.06667    33.26667 9.433333
#> 28  4     250   F         0 250        0  1.50 42.03333    30.03333 8.066667
#> 36  5     238   F         0 238        0  2.39 43.46667    33.63333 9.166667
#> 56  8     240   F         0 240        0  1.80 41.73333    31.46667 8.033334
#>    birthweight_centile         svn       lhaz        stunting stunting_outliers
#> 1          0.010765424    Term SGA -3.5406446 stunting_severe   stunting_severe
#> 28         0.002833163 Preterm SGA -2.2854751        stunting          stunting
#> 36         0.756367868 Preterm AGA -0.6087434    not_stunting      not_stunting
#> 56         0.126440075 Preterm AGA -1.6989568    not_stunting      not_stunting

When using classify_growth(), you will be informed which of the
analyses you wanted to run were successful. In the example below,
because lenht_cm is not specified, stunting indicators cannot be
computed.

life6mo_classified <- classify_growth(
  .data = life6mo_newborns,
  gest_days = gestage,
  age_days = age_days,
  sex = as.character(sex),
  weight_kg = wt_kg,
  id = as.factor(id),
  .outcomes = c("svn", "stunting")
)
#> ! Unused factor levels kept after small vulnerable newborn categorisation:
#> "Term AGA" and "Term LGA".
#> 
#> ── `gigs::classify_growth()` ───────────────────────────────────────────────────
#> ✔ Small vulnerable newborns: Success
#> ! Stunting: Not computed (`lenht_cm` not supplied)

head(life6mo_classified, n = 4)
#>    id gestage sex visitweek pma age_days wt_kg   len_cm headcirc_cm  muac_cm
#> 1   1     273   M         0 273        0  2.30 42.06667    33.26667 9.433333
#> 28  4     250   F         0 250        0  1.50 42.03333    30.03333 8.066667
#> 36  5     238   F         0 238        0  2.39 43.46667    33.63333 9.166667
#> 56  8     240   F         0 240        0  1.80 41.73333    31.46667 8.033334
#>    birthweight_centile         svn
#> 1          0.010765424    Term SGA
#> 28         0.002833163 Preterm SGA
#> 36         0.756367868 Preterm AGA
#> 56         0.126440075 Preterm AGA

You can also use classify_*() functions which are specific to the
growth indicator you’d like to calculate, for example classify_svn()
to get small, vulnerable newborn classifications for each infant:

# Small vulnerable newborns - note no ID parameter, as it is assumed that all
# measures are taken at birth
life6mo_svn <- classify_svn(
  .data = life6mo_newborns,
  weight_kg = wt_kg,
  gest_days = gestage,
  sex = as.character(sex)
)
#> ! Unused factor levels kept after small vulnerable newborn categorisation:
#> "Term AGA" and "Term LGA".

head(life6mo_svn, n = 4)
#>    id gestage sex visitweek pma age_days wt_kg   len_cm headcirc_cm  muac_cm
#> 1   1     273   M         0 273        0  2.30 42.06667    33.26667 9.433333
#> 28  4     250   F         0 250        0  1.50 42.03333    30.03333 8.066667
#> 36  5     238   F         0 238        0  2.39 43.46667    33.63333 9.166667
#> 56  8     240   F         0 240        0  1.80 41.73333    31.46667 8.033334
#>    birthweight_centile         svn
#> 1          0.010765424    Term SGA
#> 28         0.002833163 Preterm SGA
#> 36         0.756367868 Preterm AGA
#> 56         0.126440075 Preterm AGA

Conversion functions

Available international growth standards

gigs facilitates the proper use of international growth standards,
which are growth charts developed using international samples of healthy
singleton children born to mothers that had their health needs met
during pregnancy. They represent an international standard of ‘optimal’
growth. gigs implements international growth standards from the WHO
and INTERGROWTH-21^st project:

• ig_nbs - INTERGROWTH-21^st Newborn Size standards
  (including very preterm)

  Component standards

  Acronym	Description	Unit	x range
  wfga	weight-for-GA	kg	168 to 300 days
  lfga	length-for-GA	cm	168 to 300 days
  hcfga	head circumference-for-GA	cm	168 to 300 days
  wlrfga	weight-to-length ratio-for-GA	kg/cm	168 to 300 days
  ffmfga	fat-free mass-for-GA	kg	266 to 294 days
  bfpfga	body fat percentage-for-GA	%	266 to 294 days
  fmfga	fat mass-for-GA	kg	266 to 294 days

• ig_nbs_ext - Extended INTERGROWTH-21^st Newborn Size
  standards (including very preterm)

  Component standards

  Acronym	Description	Unit	x range
  wfga	weight-for-GA	kg	154 to 314 days
  lfga	length-for-GA	cm	154 to 314 days
  hcfga	head circumference-for-GA	cm	154 to 314 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-GA	mm	98 to 280 days
  bpdfga	biparietal diameter-for-GA	mm	98 to 280 days
  acfga	abdominal circumference-for-GA	mm	98 to 280 days
  flfga	femur length-for-GA	mm	98 to 280 days
  ofdfga	occipito-frontal diameter for-GA	mm	98 to 280 days
  efwfga	estimated fetal weight-for-GA	g	154 to 280 days
  sfhfga	symphisis-fundal height-for-GA	mm	112 to 294 days
  crlfga	crown-rump length-for-GA	mm	58 to 105 days
  gafcrl	GA-for-crown-rump length	days	15 to 95 mm
  gwgfga	gestational weight gain-for-GA	kg	98 to 280 days
  pifga	pulsatility index-for-GA		168 to 280 days
  rifga	resistance index-for-GA		168 to 280 days
  sdrfga	systolic/diastolic ratio-for-GA		168 to 280 days
  tcdfga	transcerebellar diameter-for-GA	mm	98 to 280 days
  tcdfga	GA-for-transcerebellar diameter	mm	98 to 280 days
  poffga	parietal-occipital fissure-for-GA	mm	105 to 252 days
  sffga	Sylvian fissue-for-GA	mm	105 to 252 days
  avfga	anterior horn of the lateral ventricle-for-GA	mm	105 to 252 days
  pvfga	atrium of the posterior horn of the lateral ventricle-for-GA	mm	105 to 252 days
  cmfga	cisterna magna-for-GA	mm	105 to 252 days
  hefwfga	Hadlock estimated fetal weight-for-GA	g	126 to 287 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

Usage

Conversion functions are named according to the conversion they perform.
Either they convert measured values to z-scores/centiles
(value2zscore()/value2centile()), or they generate expected values
for given z-scores/centiles (zscore2value()/centile2value()).

You tell gigs which international growth standard to use with the
family and acronym parameters. The family parameter denotes the
set of growth standards you want to use - e.g. "ig_nbs" for the
INTERGROWTH-21^st Newborn Size standards (including very
preterm). The acronym parameter describes which exact growth standard
you want out of all the growth standards in your ‘family’ of standards.

For example, to convert values to z-scores in the weight-for-GA
standard from the INTERGROWTH-21^st Newborn Size standards,
you would run value2zscore(..., family = "ig_nbs", acronym = "wfga").

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:

• Preterm infants:
  value2centile(..., family = "ig_png", acronym = "lfa")
• Term infants:
  value2centile(..., family = "who_gs", acronym = "lhfa")

If you don’t know which units are used for a given growth standard, the
report_units() function will help you. Run it with your family and
acronym combination to get help:

report_units(family = "ig_nbs", acronym = "wfga")
#> You're using "wfga" from the INTERGROWTH-21st Newborn Size Standards
#> ("ig_nbs").
#> ℹ Units for `y`: Weight (kg).
#> ℹ Units for `x`: Gestational age (days).

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...
value2zscore(y = 0.785, x = 182, sex = "F",
             family = "ig_nbs", acronym = "wfga") |>
  round(digits = 2)
#> [1] 0

# .. or for multiple inputs
value2centile(y = 0.785, x = seq(175, 196, by = 7), sex = "F",
              family = "ig_nbs", acronym = "wfga") |>
  round(digits = 2)
#> [1] 0.75 0.50 0.25 0.09

# You can also get centiles
value2centile(y = c(2.86, 3.12, 3.12, 3.43, 3.77, 4.10), x = 40, sex = "M",
              family = "ig_png",  acronym = "wfa") |>
  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...
zscore2value(z = 0, x = 182, sex = "F",
             family = "ig_nbs", acronym = "wfga") |>
  round(digits = 3)
#> [1] 0.785

# .. or for multiple inputs
zscore2value(z = 0, x = seq(182, 204, by = 7), sex = "F",
             family = "ig_nbs", acronym = "wfga") |>
  round(digits = 3)
#> [1] 0.785 0.893 1.013 1.147

# You can do the same for centiles
centile2value(p = c(0.1, 0.25, 0.5, 0.75, 0.9), x = 40, sex = "M",
              family = "ig_png", acronym = "wfa") |>
  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::zscore2value(z = z,
                                    x = gestage_range,
                                    sex = "F",
                                    family = "ig_nbs",
                                    acronym = "wfga")
               })
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)

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. No other packages (except gigs for Stata)
includes the extended versions of the INTERGROWTH-21^st
Newborn Size standards (incl. Very Preterm).

A tick (✅) indicates that all possible standards are included in a
package, a red cross (❌) indicates that these standards are completely
missing, and a warning sign (⚠️) indicates that some of these standards
are implemented but not others.

We have benchmarked these implementations against each other for
conversion of values to z-scores in the WHO Child Growth Standards and
different sets of INTERGROWTH-21^st standards. The table below
shows the relative speed of each software package when processing
100,000 inputs. The code used to generate these timings can be seen
online in the gigs benchmarking
article.

Note: zscorer is NA because we couldn’t time it for 100,000 inputs (it
takes too long).

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.

Authors + Citation

S. R. Parker Maternal, Adolescent, Reproductive, and Child Health
Centre, London School of Hygiene & Tropical Medicine

Dr L. Vesel Ariadne Labs, Brigham and Women’s Hospital, Harvard T.H.
Chan School of Public Health

Professor E. O. Ohuma Maternal, Adolescent, Reproductive, and Child
Health Centre, London School of Hygiene & Tropical Medicine

Citation

Parker SR, Vesel L, Ohuma EO (2025). gigs: A package for standardising
fetal, neonatal, and child growth assessment. Journal of Open Source
Software, 10(106):7655. doi:
10.21105/joss.07655.

Code of Conduct

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

References

• INTERGROWTH-21^st Newborn Size standards (including very
  preterm)
  • Villar J, Cheikh Ismail L, Victora CG, Ohuma EO, Bertino E, Altman
    DG, et al. International standards for newborn weight, length, and
    head circumference by gestational age and sex: the Newborn
    Cross-Sectional Study of the INTERGROWTH-21st Project. Lancet
    2014, 384(9946):857-68. doi:
    10.1016/S0140-6736(14)60932-6
  • Villar J, Giuliani F, Fenton TR, Ohuma EO, Ismail LC, Kennedy SH et
    al. INTERGROWTH-21st very preterm size at birth reference
    charts. Lancet 2016, 387(10021):844-45. doi:
    10.1016/S0140-6736(16)00384-6
  • Villar J, Puglia FA, Fenton TR, Ismal LC, Staines-Urias E, Giuliani
    F, et al. Body composition at birth and its relationship with
    neonatal anthropometric ratios: the newborn body composition study
    of the INTERGROWTH-21st project. Pediatric Research 2017,
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    10.1038/pr.2017.52
• INTERGROWTH-21^st Postnatal Growth of Preterm Infants
  standards
  • Villar J, Giuliani F, Bhutta ZA, Bertino E, Ohuma EO, Ismail LC et
    al. Postnatal growth standards for preterm infants: the Preterm
    Postnatal Follow-up Study of the INTERGROWTH-21st Project. Lancet
    Glob Health 2015, 3(11):e681-e691. doi:
    10.1016/S2214-109X(15)00163-1
• INTERGROWTH-21^st Fetal standards
  • Papageorghiou AT, Ohuma EO, Altman DG, Todros T, Cheikh Ismail L,
    Lambert A et al. International standards for fetal growth based on
    serial ultrasound measurements: the Fetal Growth Longitudinal Study
    of the INTERGROWTH-21st Project. Lancet 2014,
    384(9946):869-79. doi:
    10.1016/S0140-6736(14)61490-2
  • Stirnemann J, Villar J, Salomon LJ, Ohuma EO, Lamber A, Victoria CG
    et al. International Estimated Fetal Weight Standards of the
    INTERGROWTH-21st Project. Ultrasound Obstet Gynecol 2016,
    49:478-486 doi:
    10.1002/uog.17347
  • Stirnemann J, Salomon LJ, Papageorghiou AT. INTERGROWTH-21st
    standards for Hadlock’s estimation of fetal weight. Ultrasound
    Obstet Gynecol 2020, 56(6):946-948 doi:
    10.1002/uog.22000
  • Papageorghiou AT, Ohuma EO, Gravett MG, Lamber A, Noble JA, Pang R
    et al. International standards for symphysis-fundal height based
    on serial measurements from the Fetal Growth Longitudinal Study of
    the INTERGROWTH-21st Project: prospective cohort study in eight
    countries. BMJ 2016, 355:i5662 doi:
    10.1136/bmj.i5662
  • Papageorghiou AT, Kennedy SH, Salomon LJ, Ohuma EO, Cheikh Ismail L,
    Barros FC et al. International standards for early fetal size and
    pregnancy dating based on ultrasound measurement of crown-rump
    length in the first trimester of pregnancy. Ultrasound Obstet
    Gynecol 2014, 44(6):641-48 doi:
    10.1002/uog.13448
  • Cheikh Ismail L, Bishop DC, Pang R, Ohuma EO, Kac G, Abrams B et al.
    Gestational weight gain standards based on women enrolled in the
    Fetal Growth Longitudinal Study of the INTERGROWTH-21st Project: a
    prospective longitudinal cohort study. BMJ 2016, 352:i555
    doi: 10.1136/bmj.i555
  • Drukker L, Staines-Urias E, Villar J, Barros FC, Carvalho M, Munim S
    et al. International gestational age-specific centiles for
    umbilical artery Doppler indices: a longitudinal prospective cohort
    study of the INTERGROWTH-21st Project. Am J Obstet Gynecol 2021,
    222(6):602.e1-602.e15 doi:
    10.1016/j.ajog.2020.01.012
  • Rodriguez-Sibaja MJ, Villar J, Ohuma EO, Napolitano R, Heyl S,
    Carvalho M et al. Fetal cerebellar growth and Sylvian fissure
    maturation: international standards from Fetal Growth Longitudinal
    Study of INTERGROWTH-21st Project Ultrasound Obstet Gynecol
    2021, 57(4):614-623 doi:
    10.1002/uog.22017
  • Napolitano R, Molloholli M, Donadono V, Ohuma EO, Wanyonyi SZ, Kemp
    B et al. International standards for fetal brain structures based
    on serial ultrasound measurements from Fetal Growth Longitudinal
    Study of INTERGROWTH-21st Project Ultrasound Obstet Gynecol
    2020, 56(3):359-370 doi:
    10.1002/uog.21990
• WHO Child Growth Standards
  • de Onis M, Garza C, Victora CG, Onyango AW, Frongillo EA,
    Martines J. The WHO Multicentre Growth Reference Study: planning,
    study design, and methodology Food Nutr Bull. 2004, 25(1
    Suppl):S15-26. doi:
    10.1177/15648265040251s104
  • World Health Organisation. WHO child growth standards:
    length/height-for-age, weight-for-age, weight-for-length,
    weight-for-height and body mass index-for-age: methods and
    development. Technical report, WHO, Geneva, 2006.
  • World Health Organisation. WHO child growth standards: head
    circumference-for-age, arm circumference-for-age, triceps
    skinfold-for-age and subscapular skinfold-for-age: methods and
    development. Technical report, WHO, Geneva, 2007.