Pregnancy and the early post partum period are widely understood as a critical period for the infant's emotional development and the earliest influence shaping social interaction. The present study aims to understand the potential influence of both antenatal and postnatal maternal anxiety and depressive symptoms on socio-emotional outcomes in offspring aged 12 months. The study used longitudinal data from a prospective cohort study on Australian pregnant women and their children. Data were available for 282 mothers and their children. Maternal depressive and anxiety symptoms were measured in early pregnancy, trimester three of pregnancy, six and 12 months postpartum. Social and emotional development in children was measured using the Brief Infant and Toddler Social Emotional Assessment (BITSEA) at 12 months. Using growth curve analysis of 4 waves of repeated measurement to examine intercept and slope, we found that both initial maternal depression and anxiety symptom levels, and the growth of these symptoms over time, predicted more problems with children's social and emotional development. In the final model anxiety accounted for 19% of the variance in child socio-emotional problems and depression 23% of variance. The results emphasise the importance of perinatal maternal mental health as a potential risk factor for child development. This carries important implications for policy development, such as the need to build early identification and early intervention models in to the current clinical practice for perinatal care, specifically, to develop targeted screening, assessment and interventions to address maternal mental health issues for at-risk parents during pregnancy, and continuing monitoring of young children whose mothers have experienced perinatal mental health difficulties.