Maternal mental health problems are among the most common morbidities in pregnancy and postpartum with up to 25% of women experiencing depression, anxiety or stress. Systematic reviews support a clear association between poor prenatal and postnatal mental health and sub optimal child development. This suggests that the impact of maternal mental health on child outcomes is not isolated within high risk groups, but is a relevant issue in the general population. Existing literature on maternal mental health has focused on depression or anxiety during pregnancy or during the postnatal period and its associations with child outcomes. However, few studies have investigated the relationship between maternal mental health over time and child outcomes in a large population cohort.