Mental and behavioural disorders are common during childhood and adolescence and there is evidence that environmental factors and in particular the intrauterine environment plays a significant role in mental health. Long-term impact on brain development through critical central pathways and neuroanatomic changes has been shown, and factors in early life may affect predisposition to mental disorders such as depression, attention deficit/hyperactivity disorder and autism spectrum disorder (ASD). Since there is strong evidence from several different lines of research that ASD is influenced by prenatal factors, and thus the pathway to these disorders may begin in utero, maternal diet represents a strong and biologically plausible potential risk factor for ASD. We propose a prospective association study based on the Danish National Birth Cohort, which includes data from more than 100,000 pregnancies recruited early in pregnancy from 1996-2002; the first large study of its kind. We will examine the associations between selected nutrients: folates (incl. folic acid), PUFAs, vitamin D, and child risk of being diagnosed with ASD. The Danish National Birth Cohort represents a unique opportunity to conduct such a study since analyses within the cohort can utilize not only prospectively collected, appropriately timed exposure information, but also will be well-powered to detect associations.