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-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 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.
# You can install the development version from GitHub with `remotes`:
# install.packages("remotes")
remotes::install_github(repo = "lshtm-gigs/gigs")GIGS operates on anthropometric measurements, 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. In gigs, these are represented as a value between
0and1. For example,0.5corresponds to the 50th centile (i.e. the mean), whereas0.75corresponds to the 75th centile.
In growth data, z-scores and centiles represent the size a fetus, newborn, or child relative to its peers. Its size is considered relative to some 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.
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.
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")
)
#>
#> ── `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_stuntingWhen 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")
)
#>
#> ── `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 AGAYou 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)
)
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 AGAGIGS 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-21st project:
-
ig_nbs- INTERGROWTH-21st Newborn Size standards (including very preterm)Component standards
Acronym Description Unit xrangewfgaweight-for-GA kg 168 to 300 days lfgalength-for-GA cm 168 to 300 days hcfgahead circumference-for-GA cm 168 to 300 days wlrfgaweight-to-length ratio-for-GA kg/cm 168 to 300 days ffmfgafat-free mass-for-GA kg 266 to 294 days bfpfgabody fat percentage-for-GA % 266 to 294 days fmfgafat mass-for-GA kg 266 to 294 days -
ig_png- INTERGROWTH-21st Postnatal Growth of Preterm Infants standardsComponent standards
Acronym Description Unit xrangewfaweight-for-age kg 27 to ≤64 exact weeks lfalength-for-age cm 27 to ≤64 exact weeks hcfahead circumference-for-age cm 27 to ≤64 exact weeks wflweight-for-length kg 35 to 65 cm -
ig_fet- INTERGROWTH-21st Fetal standardsComponent standards
Acronym Description Unit xrangehcfgahead circumference-for-GA mm 98 to 280 days bpdfgabiparietal diameter-for-GA mm 98 to 280 days acfgaabdominal circumference-for-GA mm 98 to 280 days flfgafemur length-for-GA mm 98 to 280 days ofdfgaoccipito-frontal diameter for-GA mm 98 to 280 days efwfgaestimated fetal weight-for-GA g 154 to 280 days sfhfgasymphisis-fundal height-for-GA mm 112 to 294 days crlfgacrown-rump length-for-GA mm 58 to 105 days gafcrlGA-for-crown-rump length days 15 to 95 mm gwgfgagestational weight gain-for-GA kg 98 to 280 days pifgapulsatility index-for-GA 168 to 280 days rifgaresistance index-for-GA 168 to 280 days sdrfgasystolic/diastolic ratio-for-GA 168 to 280 days tcdfgatranscerebellar diameter-for-GA mm 98 to 280 days tcdfgaGA-for-transcerebellar diameter mm 98 to 280 days poffgaparietal-occipital fissure-for-GA mm 105 to 252 days sffgaSylvian fissue-for-GA mm 105 to 252 days avfgaanterior horn of the lateral ventricle-for-GA mm 105 to 252 days pvfgaatrium of the posterior horn of the lateral ventricle-for-GA mm 105 to 252 days cmfgacisterna magna-for-GA mm 105 to 252 days hefwfgaHadlock estimated fetal weight-for-GA g 126 to 287 days -
who_gs- WHO Child Growth Standards for term infantsComponent standards
Acronym Description Unit xrangewfaweight-for-age kg 0 to 1856 days bfaBMI-for-age kg/m2 0 to 1856 days lhfalength/height-for-age cm 0 to 1856 days hcfahead circumference-for-age cm 0 to 1856 days wflweight-for-height kg 45 to 110 cm wfhweight-for-length kg 65 to 120 cm acfaarm circumference-for-age cm 91 to 1856 days ssfasubscapular skinfold-for-age mm 91 to 1856 days tsfatriceps skinfold-for-age mm 91 to 1856 days
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 tells gigs
which set of standards you want to use - e.g. "ig_nbs" for the
INTERGROWTH-21st 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-21st 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-21st 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).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 0.90These 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 4.11We 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 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 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 (
| 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 |
| AGD | R | ✅ | ❌ | ❌ | ❌ | Values ↔ z-scores |
| childsds | R | ✅ | ❌ | ❌ | ❌ | Values → z-scores/centiles |
| ki-tools/growthstandards | R | ✅ | ✅ | Values ↔ z-scores/centiles | ||
| nutriverse/intergrowth | R | ❌ | ❌ | ❌ | Values → z-scores/centiles | |
| gigs (Stata) | Stata | ✅ | ✅ | ✅ | ✅ | Values ↔ z-scores/centiles |
| zanthro (Stata) | 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 and different sets of INTERGROWTH-21st standards. The table below shows 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.
| Software | Platform | WHO (0-5 years) (ms) | IG-21st NBS (ms) | IG-21st PNG (ms) | IG-21st Fetal (ms) |
|---|---|---|---|---|---|
| gigs | R | 100 | 80 | 20 | 8 |
| anthro | R | 2211 | ❌ | ❌ | ❌ |
| AGD | R | 119 | ❌ | ❌ | ❌ |
| childsds | R | 125 | ❌ | ❌ | ❌ |
| ki-tools/growthstandards | R | 88 | 72 | 43 | 10 |
| nutriverse/intergrowth | R | ❌ | ❌ | ❌ | 16 |
| sitar | R | 46 | ❌ | ❌ | ❌ |
| zscorer | R | NA | ❌ | ❌ | ❌ |
| gigs (Stata) | Stata | 405 | 471 | 164 | 93 |
| zanthro (Stata) | Stata | 2046 | ❌ | ❌ | ❌ |
Note: zscorer is NA because we couldn’t time it for 100,000 inputs (it
takes too long).
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.
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
Parker SR, Vesel L, Ohuma EO (2023). gigs: Assess Fetal, Newborn, and Child Growth with International Standards. https://github.com/lshtm-gigs/gigs/.
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.