## Methods

### Mortality rates

Calculation of stillbirth rate

The stillbirth rate is calculated as the proportion of births in a specified population which are stillbirths. This proportion is expressed in relation to all births.

Stillbirth rate = Number of stillbirths x 1,000 / Total number of births

Calculation of neonatal mortality rate

The neonatal mortality rate is calculated as the proportion of births in a specified population which are live born and subsequently die (neonatal deaths). This proportion is expressed in relation to all live births.

Neonatal mortality rate = Number of neonatal deaths x 1,000 / Number of live births

Calculation of perinatal mortality rate

The perinatal mortality rate is calculated as the proportion of births in a specified population which are stillbirths or neonatal deaths (perinatal deaths). This proportion is expressed in relation to all births.

Perinatal mortality rate = Number of perinatal deaths x 1,000 / Total number of births

### Perinatal mortality risk

The gestational age-specific risk of perinatal mortality is the chance of a perinatal death occurring within a specified gestation interval. This is calculated by dividing the number of perinatal deaths occurring within the gestational interval (numerator) by the total number of unborn babies at the start of the interval (denominator). The perinatal mortality risk is expressed as the proportion per 1,000 fetuses remaining in utero.

While closely related to the perinatal mortality rate as a measure of risk, the perinatal mortality risk examines the chance of a perinatal death occurring at the specified gestational interval, rather than the total number of perinatal deaths per 1,000 births (the rate) that have occurred.

### Calculation of 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.

Data used to assign percentile is supplied in the supplementary tables.

### Geography

Geographic data are based on the usual residence of the mother. In 2015 and 2016, 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 2011 for all states and territories except for the Australian Capital Territory. For the Australian Capital Territory in 2015, data by remoteness, socioeconomic status and Statistical Area Level 3 are based on Statistical Local Area of the Australian Standard Geographical Classification Edition 2011 and data by Primary Health Network are based on postcode. In 2016 the Australian Capital Territory supplied SA2 of usual residence.

### 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 the Australian Statistical Geography Standard (ASGS): Volume 5—Remoteness Structure, July 2011 for more information of remoteness.

### 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 2011 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, 2011  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) (postcode for the Australian Capital Territory in 2015 only) were linked to 2015 PHNs using Australian Bureau of Statistics correspondence files.

### Confidentiality

To maintain privacy and confidentiality of individuals, cells in the supplementary data tables are suppressed if there is a risk of disclosure of an attribute of an individual that was not already known. A cell in a table is considered identifiable if, as well as being able to identify the entity, other details are also revealed. It is AIHW policy that these cells need to be confidentialised, unless the attribute that would be disclosed is deemed to be non-sensitive in the context of the data being published.