Studies examining mechanisms underlying associations between maternal depression and adverse child outcomes (including behaviour, socioemotional adjustment, and emotion regulation) indicate that during pregnancy, maternal depression could affect child outcomes through altered placental function, epigenetic changes in the child, and stress reactivity. Infection and dietary deficiencies in the mother and the child, together with the child's genetic vulnerability, might also affect outcome. Postnatally, associations between maternal depression and child outcome are influenced by altered mother-child interactions, sociodemographic or environmental influences, and social support. Knowledge is scarce on mechanisms in low-income and middle-income countries where maternal depression is highly prevalent, and stressful factors that influence the development of perinatal maternal depression and adverse child outcome (eg, food insecurity, perinatal infections, crowded or rural living conditions, and interpersonal violence) are both more intense and more common than in high-income countries. We reviewed evidence and use the biopsychosocial model to illustrate risk factors, mediators and moderators underlying associations between maternal depression and child outcomes in low-income and middle-income countries.