Body mass index

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 2018).

BMI is a ratio of weight and height (kg/m2). The normal range of BMI for non-pregnant women is 18.5 to 24.9. While increases in BMI are expected during pregnancy, a BMI of 30 or more at the first antenatal visit is defined as obesity in pregnancy.

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

The figure shows a bar chart for body mass index by a range of topics for 2019 and a line graph of topic trends between 2012 and 2019. In 2019, 138,445 women, or nearly half of mothers, were overweight or obese.

In 2019, nearly half (47.5%) of mothers were overweight or obese. The proportion of mothers who were overweight or obese was highest among mothers aged 40 or over (52.5%).

The proportion of overweight or obese mothers increased with the number of previous pregnancies. Nearly 2 in 5 (39%)  mothers who had 4 or more previous pregnancies were obese compared with about 1 in 5 (21%) mothers with only one previous pregnancy.

Mothers who were overweight or obese were more likely to have a caesarean section birth (53.8%), than have a non-instrumental vaginal birth (45.4%).


RCOG (Royal College of Obstetricians and Gynaecologists) 2018. Care of women with obesity in pregnancy. Green-top Guideline No. 72. London: RCOG. Viewed 18 January 2021.