ABSTRACT Globally, about 20% of children are exposed to maternal depression in the first years of life, increasing risk for poor physical and developmental outcomes over the life course. Identifying interventions that can mitigate the negative consequences of maternal depression on children is key to reducing health disparities and is therefore a major public health priority. However, measurable impacts on child development may not emerge immediately after an intervention and so there is a great need to identify interim indicators of effect. Such indicators would provide early information about the intervention’s causal impact, as well as point to the biological mechanism through which these interventions shape future child development. The HPA axis is a robust marker of biological responses to stress, and there is growing evidence linking HPA axis biomarkers to both maternal depression and infant brain development. Using a cluster Randomized Control Trial in Pakistan, this study leverages novel hair-derived biomarker data (cortisol and DHEA), from 104 mother-infant dyads, to determine if treating maternal depression can alter infant HPA axis function, and to examine to what extent infant HPA axis function predicts future development outcomes. The trial’s intervention is a peer-delivered version of the Thinking Healthy Programme provided to women identified as depressed in their 3rd trimester. The study sample also includes a subset of women who were not depressed during pregnancy, as a low-risk comparison group. The mother-child dyads are being followed through 36 months post-partum, and the resulting dataset is rich with data related to mothers’ health, sociodemographic factors, parenting, and child development from 3 to 36 months post-partum. The randomized design allows us to assess the intervention’s causal effects on infant cortisol and DHEA. The main outcomes of interest are child cognitive and socioemotional development indicators at 36 months of age. The specific aims of this project are to (1) evaluate the impact of a perinatal depression intervention on infant cortisol in hair; (2) examine whether infant cortisol at 12 months predicts child development at 24 and 36 months; and (3) examine the impact of the intervention on DHEA levels in infants and the link between infant DHEA at 12 months and child developmental outcomes at 24 and 36 months. The results from this study will improve our understanding of the role of maternal mental health in child development as well as help identify important indicators to improve early intervention evaluation.