gigs: Fetal, neonatal 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 weeks PMA
  lfa	length-for-age	cm	27 to <64 weeks PMA
  hcfa	head circumference-for-age	cm	27 to <64 weeks PMA
  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	grams	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
  gaftcd	gestational age-for-transcerebellar diameter	days	12-55 mm
  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	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 supply the non-fetal functions with sex data - without this, these sex-specific growth standards cannot be applied.

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.

INTERGROWTH-21^st Fetal Growth standards

This function can be used to convert between measurements and z-scores/centiles in each of the INTERGROWTH-21^st Fetal 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	<3rd
-1	Small for gestational age (SGA)	<10th
0	Appropriate for gestational age (AGA)	10th to 90th
1	Large for gestational age (LGA)	>90th

Small vulnerable newborns

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

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

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	Not stunting	-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	Not wasting	-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, clear
. keep id gestage sex visitweek pma age_days weight_g
. local 37weeks 7 * 37
. list in f/9, noobs abbreviate(10) sep(9)
 __________________________________________________________
|  id  gestage  sex visitweek   pma   age_days   weight_g  |
|   1      273    1         0   273          0   2300      |
|   1      273    1         1   280          7   2185      |
|   1      273    1         2   288         15   2325      |
|   1      273    1         4   301         28   2575      |
|   1      273    1         6   316         43   3410      |
|   1      273    1         10  344         71   4262.3333 |
|   1      273    1         14  376        103   5050      |
|   1      273    1         18  399        126   5431.6667 |
|   1      273    1         26  460        187   5835      |
 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

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(weight_g/1000, "wfga", "v2z") ///
>     if age_days == 0, ///
>     gest_days(gestage) sex(sex) sexcode(m=1, f=2)
(2,432 missing values generated)

. egen double waz_who = who_gs(weight_g/1000, "wfa", "v2z") ///
>     if age_days > 0 & gestage >= `37weeks´, ///
>     xvar(age_days) sex(sex) sexcode(m=1, f=2)
(1,360 missing values generated)

. gen pma_weeks = pma / 7
. egen double waz_png = ig_png(weight_g/1000, "wfa", "v2z") ///
>     if age_days > 0 & gestage < `37weeks´, ///
>     xvar(pma_weeks) sex(sex) sexcode(m=1, f=2)
(1,463 missing values generated)
. drop pma_weeks

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

. gen double waz = waz_who if gestage > `37weeks´
(1,508 missing values generated)
. replace waz = waz_png if gestage < `37weeks´
(1,026 real changes made)
. replace waz = waz_nbs if age_days == 0
(57 real changes made)

. list visitweek gestage pma waz_* waz in f/9, noobs sep(9)
 ____________________________________________________________________
| visitw~k  gestage  pma    waz_nbs      waz_who  waz_png        waz |
|        0      273  273  -2.298544            .        .  -2.298544 |
|        1      273  280          .    -3.028356        .  -3.028356 |
|        2      273  288          .   -3.2685948        . -3.2685948 |
|        4      273  301          .   -3.6824586        . -3.6824586 |
|        6      273  316          .   -2.7962372        . -2.7962372 |
|       10      273  344          .   -2.5495876        . -2.5495876 |
|       14      273  376          .   -2.3102437        . -2.3102437 |
|       18      273  399          .   -2.3030732        . -2.3030732 |
|       26      273  460          .   -2.8157742        . -2.8157742 |
 ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾

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_sfga() command to give us our classifications:

. use life6mo, clear
. keep if age_days == 0
(2,432 observations deleted)

. egen sfga = classify_sfga(weight_g/1000), ///
>     gest_days(gestage) sex(sex) sexcode(m=1, f=2)

. tab sfga

       sfga |      Freq.     Percent        Cum.
------------+-----------------------------------
        SGA |         39       68.42       68.42
        AGA |         16       28.07       96.49
        LGA |          2        3.51      100.00
------------+-----------------------------------
      Total |         57      100.00

Known issues and bug reporting

We 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