Maternal psychological stress is increasingly recognised as a critical environmental factor that influences fetal development and programming through the brain–gut axis. This bidirectional network, comprising neural, endocrine, immune and microbial pathways, plays a central role in how maternal stress is communicated to both the mother and fetus. During pregnancy, heightened psychosocial stress activates the HPA axis, which disrupts gut microbiota, increases intestinal permeability and promotes systemic inflammation, which can cross the placental barrier, impacting fetal neurodevelopment. This chapter explores the mechanisms through which maternal stress alters the brain–gut axis during pregnancy, focusing on microbial shifts, immune responses and neuroendocrine disruptions. It highlights the role of the maternal gut microbiota in fetal health and underscores the importance of interventions such as dietary fibre, probiotics and stress-reduction techniques to preserve gut-brain homeostasis and optimise maternal and fetal health outcomes.