This study is the first to use population-wide birth registration data spanning 18 years, linked to census records, to examine the relationship between a comprehensive set of socioeconomic factors and the birth weight of more than 1.2 million children in Australia. 

Key findings

1. Both maternal and paternal education are positively associated with children’s birth weight, with stronger associations observed for maternal education and at the lower end of the birth weight distribution. 
2. Parental income exhibits a positive but non-linear association with birth weight, with larger effects among lower-birth-weight children. 
3. Relative to children living in rental housing, those born into families with mortgaged homes have higher average birth weights, whereas those born into families that own their homes outright have lower average birth weights. However, children from home-owning families exhibit higher birth weights at the lower end of the distribution but lower birth weights at the upper end.
4. More favourable local socioeconomic conditions are positively but non-linearly associated with birth weight, with stronger associations at the lower end of the distribution.
5. Children born to mothers who migrated from low- or middle-income countries have lower birth weights than those born to mothers from high-income countries. 

From a policy perspective, the findings suggest that policies aimed at improving socioeconomic conditions may contribute to better early-life health outcomes by reducing the risk of low birth weight, with potentially long-lasting benefits across the life course. Moreover, the finding that more favourable socioeconomic conditions appear to generate larger benefits for children at the lower end of the birth weight distribution suggests that such policies may disproportionately benefit infants at greater risk of low birth weight and their families.