Maternal stress during pregnancy is a critical factor influencing foetal development and long-term health outcomes in offspring. This chapter reviews the role of maternal stress in the development of the foetal immune system, with a focus on foetal mast cells and atopic diseases such as eczema and asthma. It explores the physiological stress response, glucocorticoid regulation and how the placenta protects the foetus from overexposure to glucocorticoids. Further, it investigates how chronic maternal stress can disrupt these processes and cause immune system dysfunction, increasing the risk of developing childhood atopic diseases. An evolutionary perspective is included to examine the origins of this vulnerability. By revealing the underlying epigenetic mechanisms and target cell types, the importance of stress reduction during pregnancy can be highlighted, and an effort can be made to improve prenatal mental healthcare.