Clinical observations suggest that children born to vegan mothers may be at risk of severe neurodevelopmental impairments due to vitamin B12 (1-3), which is to be expected, as this vitamin is exclusively found in food of animal origin (4). Thus, children born to all vegetarians, not just vegans, may be at increased risk of impaired neurodevelopment if they have an insufficient intake of dietary supplements (5). As a result, it has been suggested that mothers should be warned about the impact of vegetarianism, in particular a vegan diet, during childbearing. The aim of this study was to use an existing large-scale population-based cohort to examine the association between maternal vegetarianism and the risk of impaired neurodevelopment, including more subtle impairments, in children. We initiated the present study based on the clinical observations described in Text Box 1 in order to improve the evidence for dietary counselling of pregnant vegetarians. This article is protected by copyright. All rights reserved.