Introduction

Size for gestational age is a measure of fetal growth, where small-for-gestational-age is considered an indicator of fetal growth restriction and a marker for increased fetal and infant mortality and morbidity risk, and large-for-gestational-age is considered an indicator of accelerated fetal growth and a marker for increased risk of birth complications and infant morbidity (Health Canada, 2000, 2003).

Definitions

The following definitions are from Kramer et al. (2001):


• Small for Gestational Age (SGA): Infants that are at or below the 10th percentile in birth weight, from an infant population of the same sex and gestational age.
  
  
• Large for Gestational Age (LGA): Infants that are at or above the 90th percentile in birth weight, from an infant population of the same sex and gestational age.
  
  
• Appropriate-for-Gestational-Age (AGA): A birth is considered to be appropriate for gestational age if the birth weight is between the 10th and 90th percentiles for the infant's gestational age and sex.

Gestational Age Categories

Assignment to a gestational age outcome for an infant is based on three variables that can be found in the Hospital Abstracts data:


1. sex
   
2. gestational age and
   
3. birth weight

There are 3 major categories for gestational age (GA): pre-term, post-term and term and there are 3 size (birth weight) categories: small, acceptable (appropriate) and large. This results in 9 categories for measuring Size for Gestational Age:


1. Pre-term: small, acceptable, and large
   
2. Term: small, acceptable, and large; and
   
3. Post-term: small, acceptable, and large

For summary reporting:

• small for pre-term, small for term, and small for post-term are described as small-for-gestational age (SGA) and
  
• large for pre-term, large for term and large for post-term are described as large-for-gestational age (LGA).

SAS Code and SAS Formats

The SAS code for determining Size for Gestational Age is available in the SAS code and formats section below.

NOTE: This code is based on the tables from Kramer et al. (2001) which can be found on the Government of Canada Canadian Perinatal Surveillance web site: http://www.phac-aspc.gc.ca/rhs-ssg/bwga-pnag/index-eng.php Also note, different scales are used to measure and classify male and female births.

Size for Gestational Age Outcomes

There are 10 possible values (outcomes) for the SIZE_GESTAGE variable:

'0' = '0. Missing or invalid combination'
'1' = '1. Preterm - Acceptable weight for GA'
'2' = '2. Preterm - Small weight for GA'
'3' = '3. Preterm - Large weight for GA'
'4' = '4. Term - Acceptable weight for GA'
'5' = '5. Term - Small weight for GA'
'6' = '6. Term - Large weight for GA'
'7' = '7. Post term - Acceptable weight for GA'
'8' = '8. Post term - Small weight for GA'
'9' = '9. Post term - Large weight for GA';

Each of the above outcomes can be further re-grouped to form summary groupings:

• Small for gestational age (SGA) - SIZE_GESTAGE value = 2, 5, or 8
  
• Acceptable (Appropriate) for gestational age (GA) - SIZE_GESTAGE values = 1, 4, or 7, and
  
• Large for gestational age (LGA) - SIZE_GESTAGE values = 3, 6, or 9.

NOTES:

• A SIZE_GESTAGE value = "0" is used to indicate:
  
  • records that have a gestational age less than 22 weeks or greater than 43 weeks;
    
  • records that have a birth weight less than 300 grams or greater than 9000 grams (9kg); or
    
  • records that have missing values for gestational age or birth weight.
    
    This outcome was created so that these infants would be included in the denominator of all live born births.
  
  Caution: The tables in Kramer et al (2001) do not provide lower birth weight bounds for the SGA infants. Therefore, caution should be taken when looking at infants with unreasonably low birth weights or GAs. However, the number of events will most likely be very low.

Rates

1. Small for Gestational Age Rate: the number of live births whose birth weights are below the standard 10th percentile of birth weight for gestational age expressed as a proportion of all live births.
   
   
2. Large for Gestational Age Rate: the number of live births whose birth weights are above the standard 90th percentile of birth weight for gestational age expressed as a proportion of all live births.
   
   In addition, when analyzing Size for Gestational Age it may also be of interest to look at Preterm Birth Rates separately, as preterm birth is the most important determinant of perinatal and infant mortality, and preterm birth prevention is considered the most important perinatal challenge facing industrialized countries (page 51 of the Perinatal Health Indicators for Canada).
   
   
3. Preterm Birth Rate: The number of live births with a gestational age at birth of less than 37 weeks (<259 days) expressed as a proportion of all live births.

In MCHP Research

Please read the following sections from our Deliverables for more information about MCHP research into size for gestational age:


• from the Manitoba Child Health Atlas Update deliverable by Brownell et al. (2008):
  
  • section 3.5.2 Size for Gestational Age
  
  
• from the Perinatal Services and Outcomes in Manitoba deliverable by Heaman et al. (2012):
  
  • section 6.4 Small–for–Gestational–Age
    
  • section 6.5 Large–for–Gestational–Age
  
  
• from The 2019 RHA Indicators Atlas deliverable by Fransoo et al. (2019):
  
  • section 10.4 Small for Gestational Age (SGA) Births
    
  • section 10.5 Large for Gestational Age (LGA) Births

CAUTIONS

The following cautions should be noted:

• Only liveborn singleton births should be included. Stillbirths should be excluded.
  
• An exclusion should be considered for multiple births (e.g. twins, triplets) as size for gestational age percentiles may not apply.
  
• A new version of SAS code is provided in this concept that correctly limits Size for Gestational Age calculations to newborns with gestational age between 22-43 weeks. Previous versions of SAS code had bounds between 20-50 weeks gestation, which does not align with Kramer et al. (2001). This new version also limits Size for Gestational Age calculations to newborns with birthweight between 300-9000g, with the previous version limiting birthweight to between 200-8000g.
  
• Previously published results, including the 2019 RHA Indicators Atlas, categorized the SIZE_GESTAGE (PREGOUT) variable as "0" for records with gestational age of less than 20 weeks or greater than 50 weeks or birth weight less than 200g or greater than 8000g.