Birthweight

Key findings

  • In 2017, around 20,300 (6.7%) live-born babies were of low birthweight (less than 2,500 grams).
  • Around 15% of low birthweight babies weighed less than 1,500 grams.
  • Low birthweight was higher among mothers who smoked during pregnancy (12.9%) than mothers who did not (6%).

Low birthweight is a key indicator of a baby’s immediate health and a determinant of their future health. Low birthweight babies—whose weight at birth is less than 2,500 grams—are more likely to die in infancy or to be at increased risk of illness in infancy.

Long-term health effects can include poor cognitive development and increased risk of developing chronic diseases, such as diabetes and cardiovascular disease later in life (WHO 2014). Children born with very low birthweight are especially at high risk of developmental difficulties, poor cognitive and motor skills (Scharf et al. 2016). The risk of dying is greater for babies of very low birthweight (Mayor 2016).

Evidence has found that factors influencing low birthweight include:

  • extremes of maternal age (younger than 16 or older than 40)
  • multiple pregnancy
  • obstetric complications
  • chronic maternal conditions (for example, hypertensive disorders of pregnancy)
  • infections (such as, malaria)
  • nutritional status
  • exposure to indoor air pollution
  • tobacco
  • drug use (Blencowe et al. 2019).

Research from the Maternal Health Study conducted in Victoria found that women who experienced family violence were twice as likely to give birth to babies of low birthweight as women who did not experience violence (Brown et al. 2015).

Low birthweight is closely associated with pre-term birth—almost 3 in 4 low birthweight babies were pre-term, and more than half of pre-term babies were of low birthweight in 2017 (AIHW 2019a).

Babies may also be low birthweight because they are small for gestational age, while some low-birthweight babies may be both pre-term and small for gestational age. Babies who are small for gestational age indicates a possible growth restriction within the uterus (see, Where do I find more information?).

While this section focuses on low birthweight, high birthweight is also of concern. Evidence based on data from 12 high, middle and low-income countries indicates that higher birthweight was associated with increased odds of obesity among children aged 9–11 (Qiao et al. 2015).

Box 1: Data source on low birthweight babies

Data on birthweight is sourced from the National Perinatal Data Collection (NPDC). The NPDC is a national population-based cross-sectional collection of data on pregnancy and childbirth. The data are based on births reported to the perinatal data collection in each state and territory in Australia.

How many babies are of low birthweight?

In 2017, around 20,300 (6.7% of around 303,000) liveborn babies were of low birthweight. Girls were slightly more likely to be of low birthweight than boys (7.3% compared with 6.1%, respectively). Around 15% of low birthweight babies weighed less than 1,500 grams (AIHW 2019a).

In 2017, the proportion of low birthweight babies was higher among twins (55%) and other multiple birth babies (99%) compared with singletons (5.2%). It was also higher among mothers who smoked during pregnancy (13%) than mothers who did not (6.0%) (AIHW 2019a).

In 2017, the proportion of pre-term babies of low birthweight was considerably higher than that of full-term babies (57% and 2.2%, respectively).

Have low birthweight rates changed over time?

The proportion of liveborn low birthweight babies was fairly stable in the 11 years to 2017 ranging between 6.1% and 6.7% (Figure 1). The proportion of pre-term babies of low birthweight ranged between 56% and 58% during this time, while for full-term babies the rate ranged between 1.9% and 2.2%.

Figure 1: Low birthweight liveborn babies, 2006 to 2017

This line graph shows the percentage of pre-term and full-term low birthweight babies born each year was stable between 2006 and 2017.

Chart: AIHW. Source: AIHW NPDC.

Are low birthweight rates the same for everyone?

Low birthweight rates vary across some population groups. In 2017, babies born in:

  • Very remote areas (11%) were more likely to be of low birthweight than those born in Major cities (6.5%).
  • areas of greatest socioeconomic disadvantage were also more likely to be of low birthweight (7.8%) than those born in areas of least disadvantage (5.7%) (Figure 2).

Differences were also evident between babies of Indigenous mothers and those of non-Indigenous mothers (13% and 6.4% born of low birthweight, respectively). The proportion of low-birthweight babies born to Indigenous mothers remained relatively stable between 2006 (12.4%) and 2017 (12.5%). See also Indigenous children.

Not all these categories are mutually exclusive. It is likely that some of these influencing factors overlap.

Figure 2: Low birthweight babies by selected population groups, 2016

This bar chart shows the percentage of low birthweight babies in different population groups. The groups include Indigenous status and birthplace of the mother, remoteness area and socioeconomic area.

Chart: AIHW. Source:  AIHW NPDC.

How does Australia compare internationally?

Internationally, Australia’s proportion of low birthweight babies was equal to the Organisation for Economic Co-operation and Development (OECD) average (6.5%). The proportion of low birthweight babies was lowest in Finland and Estonia (4.1% each), and highest in Greece and Japan (9.4% each) (OECD 2018).

Figure 3: Low birthweight babies by selected OECD countries and rankings, 2016 or latest available

This bar chart shows the percentage of low birthweight babies by selected OECD countries, and their respective rankings. Finland and Estonia tied for first place (4.1%25 of live births), followed by Latvia (4.3%25). Australia ranked 19th with 6.5%25 of live births.

Notes: Data for Canada refer to 2014, and for Australia, to 2015. Exact definitions of low birthweight and of live births may differ slightly across countries. Data in this graph are for OECD member states only. Graph reflects top 3 countries, English-speaking background countries, and bottom ranked countries.

Chart: AIHW. Source: OECD Family Database.

Data limitations and development opportunities

While low birthweight is both a national and international indicator of infant health, it does have limitations as birthweight alone does not account for differentiation in growth status and maturity. Pre-term babies are inherently of low birthweight but may be of normal birthweight for their gestational age (AIHW 2019b).

Where do I find more information?

For more information on: