Concept: Size for Gestational Age

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 (i.e.: pregout) is based on three variables that can be found in the Hospital Abstracts data:

1. sex
   
2. gestational age
   
3. birth weight

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

1. small for pre-term,
2. acceptable for pre-term,
3. large for pre-term,
4. small for term,
5. acceptable for term,
6. large for term,
7. small for post-term,
8. acceptable for post-term, and
9. large for post-term.

Very often small for pre-term, small for term, and small for post-term are rolled up into "small-for-gestational age (SGA)" whereas large for pre-term, term and post-term are rolled up into "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 at The Canadian Perinatal Surveillance web site: http://www.phac-aspc.gc.ca/rhs-ssg/bwga-pnag/index-eng.php

SAS Formats

proc format;
value $pregl.;

'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. PostTerm-Acceptable weight for GA'
'8' = '8. PostTerm-Small weight for GA'
'9' = '9. PostTerm-Large weight for GA';
run;

NOTE:

• Pregout = "0" is used to capture those babies that have an unreasonable birth weight or gestational age (ie. if (nbgestpb < 20 or nbgestpb > 50) or (nbweight < 300 or nbweight > 9000).
  
• This outcome was created so that these infants would be included in the denominator since the denominator consists of all live born births.
  
• 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.

Each of the above outcomes can be further re-grouped to form SGA, Acceptable (Appropriate) for GA and LGA.

• Size for GA is SMALL when pregout in (2, 5, 8)
  
• Size for GA is ACCEPTABLE (APPROPRIATE) when pregout in (1, 4, 7)
  
• Size for GA is LARGE when pregout in (3, 6, 9)

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 (SGA) Births