Observational epidemiological studies have suggested that adverse intrauterine environments may contribute to poor psychiatric outcomes in offspring, including neurodevelopmental difficulties and symptoms of depression and anxiety. However, whether these associations reflect causal intrauterine effects remains unclear. This study aims to investigate the relationship between intrauterine growth, proxied by birth weight and gestational age, and offspring outcomes of anxiety, depression, and neurodevelopmental disorders using data from the Norwegian Mother, Father and Child Cohort Study (MoBa), the world's largest population-based pregnancy cohort with parent-offspring genotype data. Building on recent advances in partitioning genetic effects on offspring phenotypes into maternal and offspring components, we will use maternal genetic variants associated with offspring birth weight and gestational age as proxies for intrauterine effects. We will perform Mendelian randomization analyses to assess the relationship between maternal genetic effects on intrauterine growth and offspring outcomes of anxiety, depression, and neurodevelopmental disorders, while conditional on parental and offspring genotypes to reduce any bias. This study aims to utilise statistical genetics methods to examine intrauterine effects on offspring outcomes and help understand the developmental origins of mental health and neurodevelopmental disorders.