To date, this is the first population-based study to examine the recurrence risk for autism spectrum disorders (ASDs), including time trends, and the first study to consider the ASDs recurrence risk for full- and half-siblings.Objectives
To estimate the relative recurrence risk for ASDs in a Danish population, including recurrence in full- and half-siblings, and to examine time trends in ASDs relative to the recurrence risk.Design, Setting, and Participants
Population-based cohort study in Denmark. All children (about 1.5 million) born in Denmark between January 1, 1980, and December 31, 2004, were identified and followed up to December 31, 2010. We identified a maternal sibling subcohort derived from mothers with at least 2 children and a paternal sibling subcohort derived from fathers with at least 2 children.Exposures
Children having an older sibling with ASDs are compared with children not having an older sibling with ASDs.Main Outcomes and Measures
The adjusted hazard ratio for ASDs among children having an older sibling with ASDs compared with children not having an older sibling with ASDs.Results
The overall relative recurrence risk for ASDs was 6.9 (95% CI, 6.1-7.8), and it did not change significantly over time; similar risks were observed in maternal and paternal full-siblings. The relative recurrence risks were 2.4 (95% CI, 1.4-4.1) for maternal half-siblings and 1.5 (95% CI, 0.7-3.4) for paternal half-siblings.Conclusions and Relevance
Our population-based recurrence risk estimate is lower than the recently reported estimates from clinical samples. Our results demonstrate no time trend in the ASDs recurrence risk as seen in the ASDs prevalence. The difference in the recurrence risk between full- and half-siblings supports the role of genetics in ASDs, while the significant recurrence risk in maternal half-siblings may support the role of factors associated with pregnancy and the maternal intrauterine environment in ASDs.
"Autism spectrum conditions (ASCs) are common pervasive neurodevelopmental conditions which typically present in early childhood and manifest with characteristic impairments in communication and social relationships, alongside unusually repetitive behaviours and restricted interests. Numerous studies have shown ASC to be highly heritable (Ronald and Hoekstra, 2011; Berg and Geschwind, 2012; Colvert et al., 2015), with genetic heritability estimated at 80% (Lichtenstein et al., 2010); one recent estimate suggests that siblings of people with autism are 7 times more likely to be diagnosed with an ASC than are members of the general population with no genetic relationship to an autistic proband (Grønborg et al., 2013). The complex polygenic interactions underlying ASC give rise to a continuous spectrum of subclinical and clinically diagnosed presentations (Baron-Cohen et al., 2001a; Hoekstra et al., 2007). "
"What studies are needed to determine if exposure to AV materials contributes to the development of ASD and how much exposure is detrimental? Siblings of children with ASD have a significantly higher risk of developing ASD  . The authors suggest that ASD sibling studies include parental documentation of infant audiovisual viewing data that could be analyzed prospectively with respect to ASD outcome. "
"If the scientists believed that downward trend between 1999 and 2001 was caused by some phenomenon unrelated to the phaseout of the TCVs, these scientists should have included those data and then explained the trend within the discussion of the data. If the 2001 data had been included in the final publication , the results would have been consistent with a more recent CDC study  where a decreasing trend of autism prevalence in Denmark after the removal of Thimerosal in 1992 was reported. Instead of large increases in autism prevalence after 1992, the recent Danish study revealed that the autism spectrum disorder prevalence in Denmark fell steadily from a high of 1.5% in 1994-95 (when children receiving Thimerosal-free formulations were too young to receive an autism diagnosis and, because of the known offset in diagnosis, most of those being diagnosed had been born 4 to 8 years earlier [from 1985 to 1990]) to a low of 1.0% in 2002–2004 (more than 10 years after the phasein of the use of Thimerosal-free vaccine formulations was started in 1992). "
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