Australia's mothers and babies

Obesity in pregnancy contributes to increased risks of illness and death for both mother and baby. Pregnant women who are obese have an increased risk of thromboembolism, gestational diabetes, pre-eclampsia, post-partum haemorrhage (bleeding) and wound infections. They are also more likely to deliver via caesarean section. Babies of mothers who are obese have higher rates of congenital abnormality, pre-term birth, stillbirth and neonatal death than babies of mothers who are not obese (RCOG 2022).

Body mass index (BMI) is a ratio of height and weight and is calculated by dividing a person’s weight in kilograms by the square of their height in metres (kg/m²). The same BMI categories are used in pregnant and non-pregnant women. A healthy range of BMI is 18.5 to 24.9. A BMI of less than 18.5 is defined as underweight; 25 to 29.9 is defined as overweight; and 30 or more is defined as obesity. Increases in BMI are expected during pregnancy.

BMI does not necessarily reflect body fat distribution or describe the same degree of fatness in different individuals. At a population level, however, it is a practical and useful measure to identify overweight and obesity (AIHW 2023).

In the NPDC, maternal BMI refers to pre-pregnancy BMI. However, source data and methods used for data collection are not uniform nationally. For example, BMI can be calculated based on self-reported height and weight or on those measured at the first antenatal visit.

Data on maternal BMI were available for mothers in all states and territories for the first time in 2016. Due to the variation in data collection methods between jurisdictions, care must be taken when making comparisons.

Between 2011 and 2023, the proportion of mothers who were underweight and the proportion of mothers who were normal weight ranged from 38% to 49%.

Over the same period, the proportion of mothers who were overweight or obese has increased (from 21% to 27% and from 16% to 23%, respectively).

Figure 1 presents trend data on the BMI of women who gave birth, by selected maternal characteristics, between 2011 (or earliest available year of data) and 2023. Select the ‘Current data’ button to view 2023 data.

In 2023, 27% of mothers were overweight and 23% were obese.

The proportion of mothers who were obese was highest amongst women:

• with a parity of 3 (34%) or 4 or more (40%)
• who lived in Inner regional or Outer regional areas (both 29%)
• who lived in the lowest socioeconomic areas (30%)
• who had a caesarean section birth (28%).

In 2023, 2.8% of mothers were underweight. The proportion was highest amongst mothers aged under 20 (11%) or 20-24 (5.1%), women who lived in Very remote areas (4.9%) and First Nations mothers (5.2%).

References

AIHW (Australian Institute of Health and Welfare) (2023) Overweight and obesity, AIHW, Australian Government, accessed 8 June 2023.

RCOG (Royal College of Obstetricians and Gynaecologists) (2022) Being overweight in pregnancy and after birth, RCOG website, accessed 16 May 2025.