README.Rmd

---
output: github_document
---

<!-- README.md is generated from README.Rmd. Please edit that file -->

```{r setup, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  fig.path = "man/figures/README-",
  out.width = "100%",
  dev.args = list(png = list(type = "cairo"))
)
intergrowth21st <- knitr::asis_output(x = "INTERGROWTH-21<sup>st</sup>")
library(gigs)
```

```{r srr-tags-1, eval = FALSE, echo = FALSE}
#' srr-tags-readme
#'
#' @srrstats {G1.1} Addressed existence of other packages/implementations in
#'   section on 'Other packages'.
#' @srrstats {EA1.0, EA1.1, EA1.2} Addressed target audience, kinds of data,
#'   and kinds of questions addressable through gigs.
```

# gigs <img src="man/figures/logo.png" align="right" height="138" />

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[![CRAN version](https://www.r-pkg.org/badges/version/gigs)](https://www.r-pkg.org/pkg/gigs)
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## 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 `r intergrowth21st` 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](#available-international-growth-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](https://www.github.com/ropensci/gigs) with `pak`, or from the
[ropensci R-universe](https://ropensci.r-universe.dev/builds):

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

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

## Rationale + terminology
```{r srr-tags-2, eval = FALSE, echo = FALSE}
#' @srrstats {G1.3} Explained z-scores and centiles prior to discussing
#'   functions which convert between these and measured values.
```
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<sup>th</sup> centile (i.e. the mean), whereas `0.75`
  corresponds to the 75<sup>th</sup> 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](https://rlang.r-lib.org/reference/args_data_masking.html), 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.
```{r examples_classification}
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")
)

head(life6mo_classified, n = 4)
```

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.
```{r examples_classification3}
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")
)

head(life6mo_classified, n = 4)
```

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:
```{r examples_classification2}
# 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)
```

## 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 `r intergrowth21st` 
project:

* `ig_nbs` - `r intergrowth21st` Newborn Size standards (including very preterm)
  <details><summary>Component standards</summary>

  | 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 |

  </details>
* `ig_nbs_ext` - Extended `r intergrowth21st` Newborn Size standards (including
  very preterm)
  <details><summary>Component standards</summary>

  | 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 |

  </details>
* `ig_png` - `r intergrowth21st` Postnatal Growth of Preterm Infants standards
  <details><summary>Component standards</summary>

  | 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           |

  </details>
* `ig_fet` - `r intergrowth21st` Fetal standards
  <details><summary>Component standards</summary>

  | 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 |

  </details>
* `who_gs` - WHO Child Growth Standards for term infants
  <details><summary>Component standards</summary>

  | Acronym | Description                  | Unit             | `x` range       |
  |---------|------------------------------|------------------|-----------------|
  | `wfa`   | weight-for-age               | kg               | 0 to 1856 days  |
  | `bfa`   | BMI-for-age                  | kg/m<sup>2</sup> | 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 |

  </details>

### 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 `r intergrowth21st` 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 *`r intergrowth21st` 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
*`r intergrowth21st` 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:

```{r report_units}
report_units(family = "ig_nbs", acronym = "wfga")
```

### Values to z-scores/centiles
These functions allow easy conversion from measured values to z-scores or
centiles for the standard used.
```{r example_v2zp}
# Convert from z-scores for individual values...
value2zscore(y = 0.785, x = 182, sex = "F",
             family = "ig_nbs", acronym = "wfga") |>
  round(digits = 2)

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

# 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)
```

### 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).
```{r example_zp2v}
# Convert from z-scores for individual values...
zscore2value(z = 0, x = 182, sex = "F",
             family = "ig_nbs", acronym = "wfga") |>
  round(digits = 3)

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

# 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)
```

#### 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`](https://ggplot2.tidyverse.org/) for
such visualisation, but do not use it here to reduce our package's dependencies.
```{r example_zp2v_curves, dev = "png", fig.alt = "A growth chart for weight against gestational age, with lines for each SD from +2 to -2."}
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

```{r arrows_and_bench_data, echo = FALSE}
ig21st <- if (knitr::is_html_output()) {
  knitr::asis_output(x = "IG-21<sup>st</sup>")
} else {
  knitr::asis_output(x = "IG-21\textsupercript{st}")
}

larr <- knitr::asis_output(x = "&larr;")
rarr <- knitr::asis_output(x = "&rarr;")
harr <- knitr::asis_output(x = "&harr;")
yes <- if (knitr::is_html_output()) {
   knitr::asis_output(x = "✅")
} else {
  knitr::asis_output(x = "✓")
}
no <- if (knitr::is_html_output()) {
  knitr::asis_output(x = "❌")
} else {
  knitr::asis_output(x = "✘")
}
half <- if (knitr::is_html_output()) {
  knitr::asis_output(x = "⚠️")
} else {
  knitr::asis_output(x = "⚠")
}

anthro <- knitr::asis_output(x = "[anthro](https://cran.r-project.org/web/packages/anthro/index.html)")
AGD <- knitr::asis_output(x = "[AGD](https://cran.r-project.org/web/packages/AGD/index.html)")
childsds <- knitr::asis_output(x = "[childsds](https://cran.r-project.org/web/packages/childsds/index.html)")
gigs_r <- knitr::asis_output(x = "[gigs](https://www.github.com/ropensci/gigs/)")
growthstans <- knitr::asis_output(x = "[ki-tools/growthstandards](https://www.github.com/ki-tools/growthstandards/)")
sitar <- knitr::asis_output(x = "[sitar](https://cran.r-project.org/web/packages/sitar/index.html)")
zscorer <- knitr::asis_output(x = "[zscorer](https://cran.r-project.org/web/packages/zscorer/index.html)")
gigs_stata <- knitr::asis_output(x = "[gigs](https://www.github.com/ropensci/gigs-stata/) (Stata)")
zanthro <- knitr::asis_output(x = "[zanthro](https://journals.sagepub.com/doi/epdf/10.1177/1536867X1301300211) (Stata)")
intergrowth <- knitr::asis_output(x = "[nutriverse/intergrowth](https://github.com/nutriverse/intergrowth/)")
gigs_sas <- knitr::asis_output(x = "[gigs](https://github.com/SASPAC/gigs/) (SAS)")

load("vignettes/articles/benchmarking.rda")
```

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, `r intergrowth21st` Newborn Size standards (including 
Very Preterm), and the `r intergrowth21st` Postnatal Growth standards for 
preterm infants. No other packages (except **gigs** for Stata) includes the
extended versions of the `r intergrowth21st` Newborn Size standards (incl. Very
Preterm).

A tick (`r yes`) indicates that all possible standards are
included in a package, a red cross (`r no`) indicates that these standards are 
completely missing, and a warning sign (`r half`) indicates that some of these
standards are implemented but not others.

| Software        | Platform | WHO (0-5 years) | `r ig21st` NBS | `r ig21st` PNG | `r ig21st` Fetal | Functionality                     |
|-----------------|----------|-----------------|----------------|----------------|------------------|-----------------------------------|
| `r gigs_r`      | R        | `r yes`         | `r yes`        | `r yes`        | `r yes`          | Values `r harr` z-scores/centiles |
| `r anthro`      | R        | `r yes`         | `r no`         | `r no`         | `r no`           | Values `r rarr` z-scores          |
| `r AGD`         | R        | `r yes`         | `r no`         | `r no`         | `r no`           | Values `r harr` z-scores          |
| `r childsds`    | R        | `r yes`         | `r no`         | `r no`         | `r no`           | Values `r rarr` z-scores/centiles |
| `r growthstans` | R        | `r yes`         | `r yes`        | `r half`       | `r half`         | Values `r harr` z-scores/centiles |
| `r intergrowth` | R        | `r no`          | `r no`         | `r no`         | `r half`         | Values `r rarr` z-scores/centiles |
| `r sitar`       | R        | `r yes`         | `r no`         | `r no`         | `r no`           | Values `r harr` z-scores/centiles |
| `r zscorer`     | R        | `r yes`         | `r no`         | `r no`         | `r no`           | Values `r rarr` z-scores/centiles |
| `r gigs_stata`  | Stata    | `r yes`         | `r yes`        | `r yes`        | `r yes`          | Values `r harr` z-scores/centiles |
| `r zanthro`     | Stata    | `r yes`         | `r no`         | `r no`         | `r no`           | Values `r rarr` z-scores/centiles |
| `r gigs_sas`    | SAS      | `r yes`         | `r yes`        | `r yes`        | `r yes`          | Values `r harr` z-scores/centiles |

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 
`r intergrowth21st` 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](https://ropensci.github.io/gigs/articles/benchmarking.html).

| Software        | Platform | WHO (0-5 years) (ms)                      | `r ig21st` NBS (ms)                    | `r ig21st` PNG (ms)                    | `r ig21st` Fetal (ms)                   |
|-----------------|----------|-------------------------------------------|----------------------------------------|----------------------------------------|-----------------------------------------|
| `r gigs_r`      | R        | `r max_len_bench[[1]][["gigs"]]`          | `r max_len_bench[[2]][["gigs"]]`       | `r max_len_bench[[3]][["gigs"]]`       | `r max_len_bench[[4]][["gigs"]]`        |
| `r anthro`      | R        | `r max_len_bench[[1]][["anthro"]]`        | `r no`                                 | `r no`                                 | `r no`                                  |
| `r AGD`         | R        | `r max_len_bench[[1]][["AGD"]]`           | `r no`                                 | `r no`                                 | `r no`                                  |
| `r childsds`    | R        | `r max_len_bench[[1]][["childsds"]]`      | `r no`                                 | `r no`                                 | `r no`                                  |
| `r growthstans` | R        | `r max_len_bench[[1]][["gs"]]`            | `r  max_len_bench[[2]][["gs"]]`        | `r max_len_bench[[3]][["gs"]]`         | `r max_len_bench[[4]][["gs"]]`          |
| `r intergrowth` | R        | `r no`                                    | `r no`                                 | `r no`                                 | `r max_len_bench[[4]][["intergrowth"]]` |
| `r sitar`       | R        | `r max_len_bench[[1]][["sitar"]]`         | `r no`                                 | `r no`                                 | `r no`                                  |
| `r zscorer`     | R        | NA                                        | `r no`                                 | `r no`                                 | `r no`                                  |
| `r gigs_stata`  | Stata    | `r max_len_bench[[1]][["gigs_stata"]]`    | `r max_len_bench[[2]][["gigs_stata"]]` | `r max_len_bench[[3]][["gigs_stata"]]` | `r max_len_bench[[4]][["gigs_stata"]]`  |
| `r zanthro`     | Stata    | `r max_len_bench[[1]][["zanthro_stata"]]` | `r no`                                 | `r no`                                 | `r no`                                  |
| `r gigs_sas`    | SAS      | `r max_len_bench[[1]][["gigs_sas"]]`      | `r max_len_bench[[2]][["gigs_sas"]]`   | `r max_len_bench[[3]][["gigs_sas"]]`   | `r max_len_bench[[4]][["gigs_sas"]]`    |

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

```{r srr-tags-3, eval = FALSE, echo = FALSE}
#' @srrstats {G1.5, G1.6} The linked article contains code for both performance
#'   and accuracy claims when comparing this package with other software
#'   packages.
```

The WHO and `r intergrowth21st` standards are also available in standalone form,
available from the [WHO
website](https://www.who.int/tools/child-growth-standards/software) and
[`r intergrowth21st`
website](https://intergrowth21.com/),
respectively. The `r intergrowth21st` 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 (2024). *gigs: Assess Fetal, Newborn, and
Child Growth*. <https://github.com/ropensci/gigs/>.

## Code of Conduct

Please note that the **gigs** package is released with a [Contributor Code of
Conduct](https://ropensci.org/code-of-conduct/). By contributing to this project
you agree to abide by its terms.

## References

```{r doi_fn, echo = FALSE}
rmd_doi <- function(doi) {
  paste0("doi: [", doi, "](https://dx.doi.org/", doi, ")") |>
    knitr::asis_output()
}
```

* INTERGROWTH-21<sup>st</sup> 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](https://dx.doi.org/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](https://dx.doi.org/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, **82:305-316.**
doi: [10.1038/pr.2017.52](https://dx.doi.org/10.1038/pr.2017.52)


* INTERGROWTH-21<sup>st</sup> 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](https://dx.doi.org/10.1016/S2214-109X(15)00163-1)


* INTERGROWTH-21<sup>st</sup> 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](https://dx.doi.org/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](https://dx.doi.org/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](https://dx.doi.org/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](https://dx.doi.org/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](https://dx.doi.org/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](https://dx.doi.org/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](https://dx.doi.org/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](https://dx.doi.org/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](https://dx.doi.org/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](https://dx.doi.org/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.