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Healthy Communities: Child and maternal health in 2009–2012 - Technical Supplement - Low birthweight, smoking during pregnancy and antenatal visits in the first trimester

Healthy Communities: Child and maternal health in 2009–2012

Low birthweight, smoking during pregnancy and antenatal visits in the first trimester

Definitions

The World Health Organization defines low birthweight as a birthweight of less than 2,500 grams.4

Smoking during pregnancy is based on women who gave birth and reported smoking at any time during the pregnancy.

An antenatal visit is an intentional encounter between a pregnant woman and a midwife or doctor to assess and improve maternal and fetal wellbeing throughout pregnancy and prior to labour. An antenatal visit does not include a visit where the sole purpose of contact is to confirm the pregnancy, or those contacts that occurred during the pregnancy that related to other nonpregnancy related issues.1

An antenatal visit in the first trimester is defined as occurring within the first 13 weeks of pregnancy.

Data source

Data for the measures of low birthweight, smoking during pregnancy and antenatal visits in the first trimester were sourced from the Australian Institute of Health and Welfare’s (AIHW) National Perinatal Data Collection (NPDC).

The NPDC is a national population-based cross-sectional data collection of pregnancy and childbirth. It includes information on both live births and stillbirths of at least 400 grams birthweight or at least 20 weeks gestation.

The NPDC data are based on births reported to the perinatal data collection in each state and territory in Australia, and are compiled and reported on annually by AIHW’s National Perinatal Epidemiology and Statistics Unit. After each birth, midwives or other staff complete a notification form using information obtained from the mother and from the hospital or other records.5 Each state and territory has its own form and/or electronic system for collecting perinatal data. The completed notifications are forwarded to the relevant state and territory health department to form the state or territory perinatal data collection.

The NPDC consists of the Perinatal National Minimum Data Set as well as some additional data items. The Perinatal National Minimum Data Set is a specification for perinatal data elements for mandatory collection and reporting at the national level and was first specified in 1997.

Geography

Data at the Medicare Local catchment level have been compiled by applying geographic concordances to the NPDC data at the Statistical Local Area (SLA) level. For records where the SLA of usual residence overlapped Medicare Local catchment boundaries, the record was proportionally attributed to each Medicare Local catchment based on the percentage of the population of the SLA in the Medicare Local catchment.

The New South Wales (NSW) perinatal data collection gives women who give birth in NSW but live in another state or territory a proxy SLA that cannot be allocated to a Medicare Local catchment of usual residence. Therefore, data for these women are excluded from the report.

Based on additional data provided by the AIHW, a footnote has been included for each measure to indicate the number of women who lived in the Australian Capital Territory but gave birth in NSW. However, no additional data could be provided for other women who lived outside of NSW but gave birth in NSW. Other jurisdictions may provide proxy SLAs for births that occurred outside the jurisdiction, but these have been estimated to be insignificant and, at most, 0.2% of a jurisdiction's total births for the period 1 January 2007 to 31 December 2011.

Aboriginal and Torres Strait Islander mothers and their babies

Data presented for Aboriginal and Torres Strait Islander mothers and their babies are influenced by the quality and completeness of recording Aboriginal and Torres Strait Islander status, which may vary across local areas. To ensure reliable reporting for Aboriginal and Torres Strait Islander mothers and their babies, data have been aggregated over a five-year period for low birthweight and smoking during pregnancy. For antenatal visits in the first trimester, data are currently only available for 2010 and 2011 so data for this indicator have been aggregated over these two calendar years for Aboriginal and Torres Strait Islander women.

Unit of measurement

Low birthweight

The percentages of live births that were of low birthweight are presented in this report at Medicare Local catchment level for:

  • Liveborn singleton babies of all women who gave birth during the three calendar years from 1 January 2009 to 31 December 2011
  • Liveborn singleton babies of Aboriginal and Torres Strait Islander women who gave birth during the five calendar years from 1 January 2007 to 31 December 2011.

Multiple births and stillbirths are excluded.

Births are attributed to the Medicare Local catchment in which the mother usually resided at the time of the birth, irrespective of where the birth occurred.

Data at Medicare Local catchment level exclude births to Australian non-residents, residents of external territories and women who could not be allocated to a Medicare Local catchment because their SLA of usual residence was not stated or was not valid.

For further information, refer to the indicator specification on METeOR.

Smoking during pregnancy

The percentages of women who gave birth and smoked during pregnancy are presented in this report at Medicare Local catchment level for:

  • All women who gave birth during the three calendar years from 1 January 2009 to 31 December 2011
  • Aboriginal and Torres Strait Islander women who gave birth during the five calendar years from 1 January 2007 to 31 December 2011.

A woman’s tobacco smoking status during pregnancy is self-reported. Women who smoked at any time during pregnancy are included.

All women who gave birth at any time during the three calendar years from 1 January 2009 to 31 December 2011 and all Aboriginal and Torres Strait Islander women who gave birth at any time during the five calendar years from 1 January 2007 to 31 December 2011 are included. Therefore, women who gave birth more than once during the relevant periods are counted for each birth.

Births include both live births and stillbirths of at least 20 weeks gestation or 400 grams birthweight.

Data are attributed to the Medicare Local catchment in which the mother usually resided at the time of the birth, irrespective of where the birth occurred.

State and territory differences in definitions and methods used for data collection affect the comparability of these data across state and territory jurisdictions and lower levels of geography within these jurisdictions. In particular, data on smoking during pregnancy are not available for women who gave birth in Victoria in 2007 or 2008. Therefore, the percentage of Aboriginal and Torres Strait Islander women who gave birth and smoked during pregnancy during the five calendar years from 1 January 2007 to 31 December 2011 does not include Aboriginal and Torres Strait Islander women who usually resided in Victoria and gave birth in Victoria in 2007 or 2008.

Data at Medicare Local catchment level exclude women whose smoking status was not stated, Australian non-residents, residents of external territories and women who could not be allocated to a Medicare Local catchment because their SLA of usual residence was not stated or was not valid.

For further information, refer to the indicator specification on METeOR.

Antenatal visits in the first trimester

The percentages of women who gave birth and had at least one antenatal visit in the first trimester are presented in this report at Medicare Local catchment level for:

  • All women who gave birth during the two calendar years from 2010 to the end of 2011
  • Aboriginal and Torres Strait Islander women who gave birth during the two calendar years from 2010 to the end of 2011.

Women who gave birth at any time during the two calendar years from 2010 to the end of 2011 are included. Therefore, women who gave birth more than once during the two-year period are counted for each birth.

Births include both live births and stillbirths of at least 20 weeks gestation or 400 grams birthweight.

State and territory differences in definitions and methods used for data collection affect the comparability of these data across state and territory jurisdictions and lower levels of geography within these jurisdictions. The following caveats apply to the data presented for 2010–2011 in the report:

  • In Western Australia, gestational age at the first antenatal visit is reported by birth hospital, therefore data may not be available for women who attend their first antenatal visit outside the birth hospital
  • In Tasmania, data on duration of pregnancy at the first antenatal visit was not reported by hospitals still using the paper-based form for collection of NPDC data, so these data should be interpreted with caution
  • In the Australian Capital Territory, the first hospital antenatal clinic visit is often reported as the first antenatal visit and, in many cases, earlier antenatal care provided by the woman’s GP is not reported.5

Data are attributed to the Medicare Local catchment in which the mother usually resided at the time of the birth, irrespective of where the birth occurred.

Data at Medicare Local catchment level exclude women whose gestation at the first antenatal visit was not stated, Australian non-residents, residents of external territories and women who could not be allocated to a Medicare Local catchment because their SLA of usual residence was not stated or was not valid.

For further information, refer to the indicator specification on METeOR.

Reliability of percentages

NPDC data presented in the report are based on administrative data and therefore are not subject to sampling error. However, when the counts on which percentages have been calculated are small, the percentages may be subject to natural random variation. To quantify the random variation associated with NPDC data, variability bands were calculated using the standard method for calculating 95% confidence intervals for percentages:

Image of formula to calculate 95% confidence interval (CI).To calculate this confidence interval requires an estimate of the standard error of a proportion. This standard error is estimated using the square root of the proportion multiplied by its complement and divided by the number of births. We convert percentages to proportions by dividing by 100. The formula for a 95 percent confidence interval is the percentage plus or minus 1.96 times the standard error multiplied by 100.

where

P = the percentage

n = the number on which the percentage is based, i.e. the denominator.

The variability bands were used to calculate the Relative Standard Error (RSE) for each estimate and this information was used as one of the criteria for suppression of estimates. However, as the variability bands have not been used for prediction or to examine changes over time, they are not included in the report or published online (data).

Suppression of estimates

Results were suppressed for confidentiality where the numerator was less than 5 and for reliability where the denominator was less than 100.

Additional suppression rules based on the limits for RSE were developed and applied to ensure robust reporting of these data at small areas. For a dichotomous proportion, RSE has been defined as the ratio of the standard error and the minimum of the estimate and its complement (100% minus estimate). Data were suppressed where the RSE was 33% or greater or the variability band width was 33% or greater.

1. Australian Institute of Health and Welfare. National Health Data Dictionary version 16. 2012 Sep 6 [cited 2014 May 29]; Cat. no. HWI 119. Available from: http://www.aihw.gov.au/publicationdetail/?id=10737422826

4. World Health Organization. International Statistical Classification of Diseases and Related Health Problems: 10th revision. Geneva: World Health Organization; 1992.

5. Li Z, Zeki R, Hilder L & Sullivan EA. Australia’s mothers and babies 2011 [Internet]. 2013 Dec 19 [cited 2014 May 26]; Perinatal statistics series no. 28. Cat. no. PER 59. Canberra: AIHW National Perinatal Epidemiology and Statistics Unit. Available from: http://www.aihw.gov.au/reports/mothers-babies/australia-s-mothers-and-babies-2011/External link, opens in a new window.