Body mass index (BMI) trajectories that improve over the lifecourse result in better cardiometabolic profiles, but only a small proportion of children of an unhealthy weight show improving BMI trajectories. This study aimed to examine the childhood factors related to diverging BMI trajectories from childhood into adulthood using data from the Childhood Determinants of Adult Health study. A convergent parallel mixed methods design was used. Quantitative data (n=2 206) came from the first (2004-06) and second (2009-11) adult follow-ups of 8 498 Australian children (7-15 years) assessed in 1985. Using BMI z-scores, group-based trajectory modelling identified five trajectory groups: Persistently Low, Persistently Average, High Decreasing, Average Increasing and High Increasing. Qualitative data (n=50) were collected from a sub-group (2016; 38-46 years). Semi-structured interviews with 6-12 participants from each BMI trajectory group focused on individual, social and environmental influences on weight, diet and physical activity across the lifecourse. Log multinomial regression modelling estimated relative risks of trajectory group membership across childhood demographic, behavioural, health, parental and school factors. Qualitative data were thematically analysed using a constant comparative approach.

Childhood factors influenced BMI trajectories. Paternal education, main language spoken, alcohol and self-rated health were significant quantitative childhood predictors of BMI trajectory. A distinct ‘legacy effect’ of parental lifestyle influences during childhood was apparent among interview participants in the Stable and High Decreasing groups, a strong and mostly positive concept discussed by both men and women in these groups and persisting despite phases of unhealthy behaviours. In contrast, the ‘legacy effect’ was much weaker in the two Increasing BMI groups. This study is the first to simultaneously identify important quantitative and qualitative childhood factors related to divergent BMI trajectories, and to observe a legacy effect of parents’ lifestyle behaviours on divergent BMI trajectories. This work provides direction for further exploration of the factors driving divergent BMI trajectories.