Methods

Crude rates

A crude rate is defined as the number of events over a specified period (for example, a year) divided by the total population exposed to the event.

Age-specific rates

An age-specific rate is defined as the number of events for a specified age group over a specified period (for example, a year) divided by the total population exposed to the event in that age group.

Age-standardised rates

Age-standardised rates enable comparisons to be made between populations that have different age structures. Direct standardisation, in which the age-specific rates are multiplied by a constant population, was used in this report. This effectively removes the influence of the age structure on the summary rate. The report states where age-standardised rates have been used.

All age-standardised rates in this report have used the June 2001 Australian female estimated resident population aged 15–44 years as the standard population. For more information refer to the Metadata Online Registry for age-standardised rates.

Geography

Geographic data are based on the usual residence of the mother. In 2018, the usual residence of the mother is based on Statistical Area Level 2 (SA2) of the Australian Bureau of Statistics Australian Statistical Geography Standard Edition 2016 for all states and territories.

Remoteness

This report uses the Australian Statistical Geography Standard Remoteness Structure which groups geographic areas into six classes of Remoteness Area based on their relative access to services using the Accessibility/Remoteness Index of Australia.

The six classes are: Major cities, Inner regional, Outer regional, Remote, Very remote and Migratory, see Australian Statistical Geography Standard (ASGS): Volume 5—Remoteness Structure, July 2016 (ABS 2018a).

Socioeconomic status

The Socio-Economic Indexes for Areas (SEIFA) are measures of socioeconomic status (SES) that summarise a range of socioeconomic variables associated with disadvantage. Socioeconomic disadvantage is typically associated with low income, high unemployment and low levels of education.

The SEIFA index used in this report is the 2016 SEIFA Index of Relative Socioeconomic Disadvantage (IRSD) developed by the Australian Bureau of Statistics for use at Statistical Area Level 2.

Since the IRSD summarises only variables that indicate disadvantage, a low score indicates that an area has many low-income families, many people with little training and many people working in unskilled occupations; hence, this area may be considered disadvantaged relative to other areas. A high score implies that the area has few families with low incomes and few people with little or no training and working in unskilled occupations. These areas with high index scores may be considered less disadvantaged relative to other areas. It is important to understand that a high score reflects a relative lack of disadvantage rather than advantage and that the IRSD relates to the average disadvantage of all people living in a geographic area and cannot be presumed to apply to all individuals living within the area.

Population-based Australian cut-offs for SEIFA quintiles have been used in this report. This method ranks the SEIFA scores for a particular geography (for example, Statistical Area Level 2) from lowest to highest, and the geographical areas are divided into 5 groups, such that approximately 20% of the population are in each group.

The most disadvantaged group is referred to as the Lowest socioeconomic status (SES) areas and the least disadvantaged group is referred to as the Highest SES areas.

See the Census of Population and Housing: Socio-Economic Indexes for Areas (SEIFA), Australia, 2016 (ABS 2018b) for further information on SEIFA.

Primary Health Network

Primary Health Networks (PHNs) have been established by the Department of Health to increase the efficiency and effectiveness of medical services and improve the coordination of care for patients.

Perinatal data at Statistical Area Level 2 (SA2) were linked to 2017 PHNs using Australian Bureau of Statistics correspondence files.

The relevant proportion for each PHN was then calculated, and categories were developed based on the median, interquartile ranges and 10th and 90th percentiles for the proportions at the PHN level. The categories were then adjusted to account for natural breaks in the distribution of the data and for easier interpretation (for example, a range with a maximum of 52.1% of mothers receiving antenatal care in the first trimester would be revised to a maximum of 50%). PHNs were allocated to categories based on unrounded proportions.

Statistical Area Level 3

Perinatal data at Statistical Area Level 2 (SA2) were linked to Statistical Area Level 3 (SA3) using Australian Bureau of Statistics correspondence files.

Small numbers

Numbers of less than 5 have not been published (n.p.), in line with guidelines for protecting the privacy of individuals. Exceptions to this are small numbers in ‘Other’ and ‘Not stated’ categories. Consequential suppression of numbers has also been applied where required to prevent back-calculation of small numbers. However, all suppressed numbers have been included in the totals.

Per cents based on denominators of less than 100 have also been suppressed (n.p.) for reliability reasons.

Australian national birthweight percentiles by gestational age

Birthweight percentiles were calculated from data on all liveborn singleton babies born in Australia between 2004 and 2013 with a gestational age of 20–44 weeks.

Records with indeterminate sex were excluded from analysis. Records with missing or not stated data for sex, birthweight or gestational age were also excluded. Birthweight outliers were calculated and excluded using a method based on Tukey’s box and whisker plots. 

Gestational age is reported in completed weeks of gestation, calculated from the first day of the last menstrual period (LMP) or estimated by prenatal and/or postnatal assessment if the LMP date was missing. Birthweight is reported to the nearest 5 grams.

Small for gestational age is defined as babies with birthweight below the 10th percentile according to the national birthweight percentiles for 2004 to 2013.

Robson 10 group classification system

The Robson 10 group classification system (Robson classification) categorises women who gave birth into 10 mutually exclusive groups (Table 3). In addition, groups 2 and 4 can be further broken down into subgroups. These subgroups are used to differentiate between women who were induced and who had a caesarean section before labour onset. 

Table 3: Robson 10 group classification system

Group Definition
1 First-time mother, singleton pregnancy, baby in cephalic (head first) presentation, ≥37 weeks gestation, spontaneous labour (not induced)
2 First-time mother, singleton pregnancy, baby in cephalic (head first) presentation, ≥37 weeks gestation, induced labour or caesarean section before labour
3 Mother has previously given birth without a previous caesarean scar, singleton pregnancy, baby in cephalic (head first) presentation, ≥37 weeks gestation, spontaneous labour (not induced)
4 Mother has previously given birth without a previous caesarean scar, singleton pregnancy, baby in cephalic (head first) presentation, ≥37 weeks gestation, induced labour or caesarean section before labour
5 Mother has previously given birth with a previous caesarean scar, singleton pregnancy, baby in cephalic (head first) presentation, ≥37 weeks gestation, induced labour or caesarean section before labour
6 First-time mother, singleton pregnancy, baby in breech (feet first) presentation
7 Mother has previously given birth with current singleton baby in breech (feet first) presentation
8 Multiple pregnancy, including women with previous caesarean scars
9 All women with a singleton pregnancy, baby in transverse (side on) or oblique lie, including women with previous caesarean scars
10 All women with a singleton pregnancy, baby in cephalic (head first) presentation, ≤36 weeks gestation, including women with previous caesarean scars

The Robson classification groups and subgroups were calculated from data on all women who gave birth in Australia for 2018. Data elements used for calculation of the groups and subgroups were parity, previous caesarean sections, onset of labour, birth plurality, gestational age, presentation at birth and method of birth.

Records for whom one or more of the following variables were not stated: parity, previous caesarean sections, onset of labour, birth plurality, gestational age and presentation at birth; were grouped into the ‘Not applicable’ category. The denominator of ‘Number of women who gave birth’ includes women with a ‘not stated’ method of birth.

The figure describes the process of categorising all women who gave birth into the 10 groups and the additional subgroups.

Process flow of classification using the Robson 10 group classification system

This figure shows the process flow for allocating women who gave birth to the Robson 10 group classification system according to their values for 6 data items: plurality, presentation, gestational age, parity, previous caesarean and labour onset.

References

ABS (Australian Bureau of Statistics) 2018a. Australian Statistical Geography Standard (ASGS): Volume 5—Remoteness Structure, July 2016. ABS cat. no. 1270.0.55.005. Canberra: ABS.

ABS 2018b. Census of Population and Housing: Socio-Economic Indexes for Areas (SEIFA), Australia, 2016. ABS cat. no. 2033.0.55.001. Canberra: ABS.

AIHW (Australian Institute of Health and Welfare) 2019. Australia’s mothers and babies 2016—in brief. Perinatal statistics series no. 34. Cat. no. PER 97. Canberra: AIHW.

COAG (Council of Australian Governments) 2012. National Healthcare Agreement 2012. Intergovernmental agreement on federal financial relations. Canberra: COAG. Viewed 31 March 2020.

WHO (World Health Organization) 1992. International Statistical Classification of Diseases and Related Health Problems, 10th Revision. Geneva: WHO.