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Researchers calculate sibling autism spectrum disorder risk

If you have an older sibling with autism then you’re more likely to be autistic yourself, the study found.

The risk of a couple who have a child with an Autism Spectrum Disorder having another child with the same condition is around 7%, according to a new Danish study released today.

The new findings, published in the journal JAMA Pediatrics, put the risk of siblings sharing Autism Spectrum Disorder (ASD) significantly lower than was previously thought.

Past estimates suggested that the recurrence risk was around 18-19%, Australian experts have said.

The new study found that the recurrence risk among a population varies between 4.5% to 10.5% depending on the birth years.

The researchers also showed that there has not been an increase in the relative recurrence risk over time, despite an increase in the overall prevalence of Autism Spectrum Disorder.

The researchers analysed data from 1.5 million Danish children born between 1980-2004. Children with autism were identified using the Danish Psychiatric Central Registry. The researchers measured the prevalence of an infant developing Autism Spectrum Disorder when that child had a older sibling already with autism.

They identified two cohort groups: a maternal sibling group derived from mothers and with at least two children and a paternal group derived from fathers with at least two children.

The study shows that a sibling of a child with Autism Spectrum Disorder has around a 7% increased risk of developing autism.

This is much higher in comparison to the Autism Spectrum Disorder risk to the general Danish population of 1.18%.

“The difference in the recurrence risk between full and half-siblings supports the role of genetics in ASDs, while the significant recurrence in maternal half siblings may support the role of factors associated with pregnancy and the maternal intrauterine environments in ASDs,” the study concludes.

“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,” the authors said.

Professor Andrew Whitehouse, a researcher of childhood health and autism from the University of Western Australia, said the new findings are important because the study examined the risk to a whole population rather than just children who are referred to researchers after being diagnosed with autism spectrum disorder.

“The estimate of 7% risk may now become the dominant figure for counselling families who have a child with autism and are looking to have another child, ” said Professor Whitehouse, who was not involved with the study.

Caution required

Dr Jon Brock, an autism expert and ARC Australian Research Fellow in Cognitive Science at Macquarie University, said the study confirmed that autism, to some extent, runs in families.

“If you have an older sibling with autism then you’re more likely to be autistic yourself. The study puts a number on this risk - about 7%. This compares with about 1% in the general population,” he said.

“It’s probably the best estimate yet, because it’s not biased by how the families were recruited for the study. The study was really only possible because of the way Denmark collects data on all its children. It’s a good example of how useful this kind of data collection is.”

The finding that recurrence risk was much lower in half siblings who had the same mother but different fathers makes sense if autism is caused by the combination of genes coming from both parents, said Dr Brock, who was not involved in the study.

“Interestingly, recurrence risk for siblings with the same father but different mothers was not much higher than in the general population. The authors of the study argue that this is consistent with the idea that pregnancy is an important time in the development of autism,” he said.

“However, I think we need to be cautious in interpreting this finding. We know that your chances of being diagnosed with autism vary a lot depending on where you live - and siblings with the same father but different mothers are probably more likely to grow up separately.”

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