We know that health behaviours during pregnancy (e.g., diet and physical activity) can impact on health outcomes (e.g., gestational diabetes). However, less is known about the impact of varying health behaviours on mental health outcomes (e.g., perinatal depression), and how the behaviour change of (reducing or increasing) these behaviours may impact perinatal mental health. This project will use traditional and causal epidemiological methods to investigate the impact of alcohol, tobacco and physical activity during pregnancy on maternal mental health using two longitudinal birth cohorts, to highlight pregnancy timepoints to reduce harm.