Autism spectrum disorders (ASDs) are developmental disorders characterized by impairments in reciprocal social interaction, communication, restricted patterns of interests and stereotyped behaviours 
. The reported prevalence of ASDs has risen steadily over the past few decades, and approximately 1% of the child population in the UK 
and the USA 
have recently been estimated to have ASDs 
. ASDs are disabling conditions and associated with significant costs to individuals, their families and society 
but their aetiology is not well understood. Although genetic factors are important, it is also acknowledged that environmental factors may play a role in the aetiology of ASD 
Maternal exposure to stressful life events during pregnancy has been suggested as a potential risk factor for ASD 
. Animal experiments have found that exposure to stress during pregnancy may adversely affect the neurodevelopment of the offspring, including in domains relevant to autism (for review see Kinney et al, 2008) 
. Mechanisms such as DNA methylation 
, or programming of the hypothalamic-pituitary-adrenal axis have been proposed as potential pathways by which psychological stress could affect neurodevelopment 
. Although biologically plausible, the evidence supporting the relationship between psychological stress in pregnancy and ASD in human studies is limited and inconsistent.
Studies supporting the role of stressful events on the risk of ASD include an ecological investigation reporting that prenatal exposure to tropical storms was associated with a higher prevalence of ASD 
; a small study (n
56) finding a higher occurrence of ‘family discord’ reported by mothers of ASD children as compared to controls at antenatal interviews 
; and a study based on maternal recall, showing that ASD mothers were more likely to report having stressful life events during pregnancy than controls 
. Furthermore, a prospective Australian study, found a small but significant association between life-event exposure during pregnancy and autistic traits in 2 year old male, but not female children as measured by a subscale of the Child Behaviour Checklist 
. In contrast, a large Danish study did not find evidence for an association between maternal exposure to bereavement during pregnancy and risk of offspring ASD 
. Although based in the general population, this study had an unusually low cumulative incidence of ASD (0.16%), and may have been prone to outcome misclassification. The evidence on the role of stressful life events in the development of ASD is therefore inconclusive.
Methodologically strong studies on this issue are required, and may help provide a better understanding of the aetiology of ASD. We used two large population-based cohorts with prospectively collected data and complementary strengths to investigate related aspects of this research question. In the first study, based in Sweden, using record linkage data on almost 4500 individuals with ASD, we investigated the relationship between the occurrence of severe, but rare life events during pregnancy and offspring ASD. In the second study, based in England, we used data on the combined occurrence of over 40 common and rare life events along with their perceived impact, and studied these in relation to the risk of offspring ASD. Life events in both studies were measured at several time points, including pregnancy and early life. We examined the hypothesis that the exposures in both cohorts would be associated with an increased risk of offspring ASD, and that these risks would be highest when the exposures occurred during the prenatal period.