gigs: Newborn and infant growth assessment in Stata

About

Produced as part of the Guidance for International Growth Standards (GIGS) 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 between 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.

Installation

The gigs package is available for Stata version 16 and over. You can install the latest stable release of gigs from GitHub using the github module for Stata:

. github install lshtm-gigs/gigs-stata

Alternatively, you can download a stable release of your choice from GitHub using the net install command from Stata. Simply go to the stable release of gigs that you want to download from the releases page on GitHub, and download the zipped archive. Unzip this downloaded archive. Within this unzipped folder will be another folder, inside which will be the .ado/.dta files needed for gigs to work. Put the path to the folder containing the .ado/.dta files in the from() option of net install, and Stata will install the necessary files.

. net install gigs, from("directory/of/unzipped/folder/with/ado/files")

Available standards

ig_nbs - INTERGROWTH-21^st standards for newborn size

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 standards for postnatal growth in preterm infants

Component standards

Acronym	Description	Unit	`xvar()` 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

who_gs - WHO Child Growth Standards for term infants

Component standards

Acronym	Description	Unit	`xvar()` 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-length	kg	45 to 110 cm
`wfh`	weight-for-height	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

Each conversion function has similar syntax. The main function call determines the set of standards in use, the acronym parameter specifies which component standard is being used, and the conversion parameter specifies the type of conversion you wish to perform. This conversion parameter can take one of four values: "v2z" (value-to-z-score), "v2c" (value-to-centile), "z2v" (z-score-to-value), "c2v" (centile-to-value). The sex() and sexcode() options are used to give the function sex data - as the growth standards are sex-specific, the standards cannot be applied correctly without this information.

INTERGROWTH-21^st Newborn Size standards, including very preterm

This function can be used to convert between measurements and z-scores/centiles in each of the INTERGROWTH-21^st Newborn Size Standards.

INTERGROWTH-21^st Postnatal Growth standards

This function can be used to convert between measurements and z-scores/centiles in each of the INTERGROWTH-21^st Postnatal Growth of Preterm Infants Standards.

WHO Child Growth Standards

This function can be used to convert between measurements and z-scores/centiles in each of the WHO Child Growth Standards.

Classification functions

These functions are used to classify infant growth according to published cut-offs. These publications are discussed in the attached paper.

Size for gestational age

This function outputs a variable with the following values and labels. Severely SGA infants are only labelled if the severe option is specified:

Value	Meaning	Centile range
-2	Severely small for gestational age	<3^rd
-1	Small for gestational age (SGA)	<10^th
0	Appropriate for gestational age (AGA)	10^th to 90^th
1	Large for gestational age (LGA)	>90^th

Small vulnerable newborns

This function outputs a variable with the following values and labels:

Value	Meaning	Term Status	Centile range
-4	Preterm SGA	Preterm	<10^th
-3	Preterm AGA	Preterm	10^th to 90^th
-2	Preterm LGA	Preterm	>90^th
-1	Term SGA	Term	<10^th
0	Term AGA	Term	10^th to 90^th
1	Term LGA	Term	>90^th

Stunting

The function outputs a variable with the following values and labels. Outlier observations are only labelled if the outliers option is specified:

Value	Meaning	Z-score range
-2	Severe stunting	-5 to -3
-1	Stunting	-3 to -2
0	Normal	-2 to 5
-10	Implausible	<-5 or >5

Wasting

The function outputs a variable with the following values and labels. Outlier observations are only labelled if the outliers option is specified:

Value	Meaning	Z-score range
-2	Severe wasting	-5 to -3
-1	Wasting	-3 to -2
0	Normal	-2 to 2
1	Overweight	2 to 5
-10	Implausible	<-5 or >5

Weight-for-age

The function outputs a variable with the following values and labels. Outlier observations are only labelled if the outliers option is specified:

Value	Meaning	Z-score range
-2	Severely underweight	-6 to -3
-1	Underweight	-3 to -2
0	Normal weight	-2 to 2
1	Overweight	2 to 5
-10	Implausible	<-6 or >5

Examples

This section illustrates a possible use case using life6mo.dta, an extract of data from the Low birthweight Infant Feeding Exploration (LIFE) Study. It contains weight measurements for term and preterm infants from birth (visitweek == 0) to around six months of age (visitweek == 26).

. use life6mo
. local 37weeks 7 * 37
. list in f/10, noobs abbreviate(10) sep(10)
 ___________________________________________________________
| infantid     gestage   pma   sex   visitweek   meaninfwgt |
|-----------------------------------------------------------|
| 101-1002-1   132       133   2     0           2300       |
| 101-1002-1   132       139   2     1           2270       |
| 101-1002-1   132       146   2     2           2465       |
| 101-1002-1   132       162   2     4           2700       |
| 101-1002-1   132       174   2     6           3160       |
| 101-1002-1   132       203   2     10          4004       |
| 101-1002-1   132       230   2     14          4893.333   |
| 101-1002-1   132       258   2     18          5690       |
| 101-1002-1   132       317   2     26          6950       |
| 101-1003-1   237       238   1     0           1900       |
|-----------------------------------------------------------|

Conversion

We can use the conversion functions listed above to generate weight-for-age z-scores (WAZs) in the different study populations (i.e. term vs preterm) and measurement timings (i.e. z-scores for newborns with INTERGROWTH-21^st Newborn Size Standards, WHO/INTERGROWTH Postnatal standards after birth).

. egen double waz_nbs = ig_nbs(meaninfwgt/1000, "wfga", "v2z") ///
>     if agedays == 0, ///
>     gest_days(gestage) sex(sex) sexcode(m=1, f=2)
(8,228 missing values generated)

. egen double waz_who = who_gs(meaninfwgt/1000, "wfa", "v2z") ///
>     if agedays > 0 & gestage >= `37weeks', ///
>     xvar(agedays) sex(sex) sexcode(m=1, f=2)
(4,073 missing values generated)


. gen pma_weeks = pma / 7
. gen pma_weeks_floored = floor(pma / 7)
. egen double waz_png = ig_png(meaninfwgt/1000, "wfa", "v2z") ///
>     if gestage < `37weeks' & agedays > 0, ///
>     xvar(pma_weeks_floored) sex(sex) sexcode(m=1, f=2)
(4,659 missing values generated)
. drop pma_weeks pma_weeks_floored

. gen age_corrected = pma - `40weeks´
. egen waz_whocorr = who_gs(meaninfwgt/1000, "wfa", "v2z") ///
>     if gestage < `37weeks´ & visitweek == 26, ///
>     xvar(age_corrected) sex(sex) sexcode(m=1, f=2)
(7,996 missing values generated)
. drop age_corrected

We can then combine these WAZs into one overall waz variable:

. gen double waz = waz_who if gestage > `37weeks'
(4,749 missing values generated)
. replace waz = waz_png if gestage < `37weeks'
(3,757 real changes made)
. replace waz = waz_nbs if agedays == 0
(188 real changes made)

. list visitweek gestage pma waz_* waz in f/10, noobs sep(10)
  +------------------------------------------------------------------------+
  | visitw~k   gestage   pma   waz_nbs   waz_who      waz_png          waz |
  |------------------------------------------------------------------------|
  |        0       132   133         .         .            .            . |
  |        1       132   139         .         .            .            . |
  |        2       132   146         .         .            .            . |
  |        4       132   162         .         .            .            . |
  |        6       132   174         .         .            .            . |
  |       10       132   203         .         .    7.8594946    7.8594946 |
  |       14       132   230         .         .    7.2548282    7.2548282 |
  |       18       132   258         .         .    6.0985216    6.0985216 |
  |       26       132   317         .         .    3.8344571    3.8344571 |
  |        0       237   238         .         .   -.43606208   -.43606208 |
  +------------------------------------------------------------------------+

This waz variable can then be used to determine whether infants are underweight at different age points, or to track the growth trajectory of individual children.

Classification

This dataset contains information on weight at birth, so could be used to calculate size-for-gestational age classifications. We can reload the data, then remove any observations which were not made at birth. We then use the classify_sga() command to give us our classifications:

. use life6mo, clear
. keep if (gestage - pma) == 0
(7,303 observations deleted)

. egen sga = classify_sga(meaninfwgt/1000), ///
>     gest_days(gestage) sex(sex) sexcode(m=1, f=2)
(2 missing values generated)

Known issues and bug reporting

We kindly request that users note any bugs, issues, or feature requests on the GitHub issues page.

Authors

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

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

lshtm-gigs / gigs-stata

gigs: Newborn and infant growth assessment in Stata

About

Installation

Available standards

Conversion functions

INTERGROWTH-21^st Newborn Size standards, including very preterm

INTERGROWTH-21^st Postnatal Growth standards

WHO Child Growth Standards

Classification functions

Size for gestational age

Small vulnerable newborns

Stunting

Wasting

Weight-for-age

Examples

Conversion

Classification

Known issues and bug reporting

Authors

About

Languages

gigs: Newborn and infant growth assessment in Stata

About

Installation

Available standards

Conversion functions

INTERGROWTH-21st Newborn Size standards, including very preterm

INTERGROWTH-21st Postnatal Growth standards

WHO Child Growth Standards

Classification functions

Size for gestational age

Small vulnerable newborns

Stunting

Wasting

Weight-for-age

Examples

Conversion

Classification

Known issues and bug reporting

Authors

About

Languages

INTERGROWTH-21^st Newborn Size standards, including very preterm

INTERGROWTH-21^st Postnatal Growth standards