The prenatal environment contributes substantially to risk for serious mental illnesses that can emerge decades after birth. As such, prenatal life may be a propitious time to intervene in ways that reduce risk. Indeed, replicated, robust evidence from recent public health studies associates early prenatal exposure to folic acid with an approximately 50% reduction in autism risk. Further, our recent work associated increased fetal exposure to population-wide folic acid fortification of grain products with specific changes in brain development, characterized by delayed thinning of the cerebral cortex during adolescence and associated reduction in risk for psychosis symptoms. These changes were most pronounced in regions subserving frontoparietal control and limbic networks, which have been broadly implicated across categories of psychiatric illness, and specifically linked to variation in blood folate levels in healthy adults and patients with schizophrenia by our group. These findings are unprecedented, and of potentially substantial