Maternal mental health during pregnancy is important for maternal wellbeing and offspring development. Prenatal depression, anxiety, stress, and psychological distress have been associated with offspring emotional, behavioural, cognitive, and mental health outcomes, but evidence remains dispersed across conditions, outcomes, and proposed mechanisms. This scoping review synthesises empirical evidence on maternal mental health during pregnancy as a potential intergenerational exposure, including potential in-utero pathways.Five databases (MEDLINE, Scopus, PsycInfo, CINAHL, and Embase) were searched from 1995 to 24 February 2026, following PRISMA-ScR guidance. Eligible sources of evidence examined maternal mental health during pregnancy in relation to postnatal offspring outcomes. Data were extracted using a structured framework and synthesised narratively. Twenty sources of evidence met the inclusion criteria. Across diverse designs and populations, prenatal depression, anxiety, stress, and psychological distress were commonly associated with adverse offspring outcomes, including emotional and behavioural difficulties, neurocognitive impairments, and increased risk of psychiatric conditions from childhood to adulthood. Several primary studies reported associations independent of traditional birth outcomes, consistent with potential in-utero pathways. Plausible mechanisms included stress-related neuroendocrine processes, inflammation, and epigenetic regulation. However, heterogeneity in exposure measurement, outcome definitions, and follow-up periods limited comparability and causal interpretation.Maternal mental health during pregnancy may be an important early-life and intergenerational exposure associated with offspring developmental trajectories, but observational evidence does not establish causal in-utero transmission. Stronger causal inference, longitudinal data linkage, and incorporation of economic and policy-relevant outcomes are priorities for future research.