Socioeconomic disadvantage and stress are associated with adverse pregnancy and child health outcomes. There is some evidence to suggest that these associations may be mediated by changes in placental development and function. However, studies of the placenta are often small or limited to pathologic samples, which may not be representative. ALSPAC routinely collected placentas of participants, including term deliveries, and ALSPAC has rich parental sociodemographic information and longitudinal follow-up on child health and development. This presents a unique opportunity to estimate associations between socioeconomic disadvantage and child development and to examine the mediating role of placental pathology in a well-characterized, representative sample.