Cognitive delay has previously been found to be associated with low birthweight. Little research has been conducted to explore the specific association between low birthweight and intellectual disability (defined as having an arrested or incomplete development of the mind alongside functional impairment in facets that contribute to overall intelligence such as cognition, language and social ability), particularly among populations that were born at an appropriate gestational age. The current project will explore the observational association in ALSPAC before going on to investigate potential moderating factors (factors that influence the strength of the effect). We will then investigate whether the association is likely to reflect a causal effect by using techniques such as Mendelian Randomization.