Vitamin D is a steroid hormone considered critical for brain development. A link between vitamin D deficiency and offspring autism has been hypothesised and may be biologically plausible. However, very few studies have tested this association and the results have been inconsistent (Whitehouse et al 2012, Magnusson et al 2016, Gale et al 2008, Vinkhuyzen et al 2016), although do suggest further work on this topic is warranted. Considering Vitamin D supplementation may be scaleable at low cost, it is important to study whether any associations between Vit D deficiency during pregnancy and adverse offspring outcomes are likely to be causal.