A large population-based study from Denmark has followed siblings for the risk for autism spectrum disorders, finding different likelihoods depending on birth year, and also whether brothers or sisters were half- or full-siblings.

The study uses records of all children born in Denmark between 1980 and 2004. It is the first study of its kind, say the authors, to follow such a large number of children – around 1.5 million – and to consider the “recurrence risk” of autism spectrum disorders (ASD) for both full- and half-siblings.

The study – published online in JAMA Pediatrics by researchers from Aarhus University – compared children who had an older sibling with ASD against those whose older sibling did not have ASD.

According to the authors, 30% of all ASD cases are childhood autism, and the prevalence of autism spectrum disorders has increased over the past 20 years.

The study shows that between 1980 and 2004, the recurrence risks for ASDs ranged between 4.5% and 10.5%, higher than the risk of autism spectrum disorders found across the Danish population, of 1.18%.

Additionally, the researchers found there was an almost seven-fold greater risk for an autism spectrum disorder if an older sibling had an ASD diagnosis, compared with families in which the older sibling did not have a disorder.

For children with the same mother, the recurrence risk was 7.5% for full-siblings and 2.4% for half-siblings.

For children with the same father, the recurrence risk was 7.4% for full-siblings, but the researchers not that there was “no statistically significant increased risk” among half-siblings.

The authors note that the reason the risk is higher for half-siblings who share a mother may be due to the fact that they share genes from their mother, and they also share “exposures derived from their mother’s intrauterine environment and perinatal history” across her different pregnancies.

An important issue to address from the study, say the researchers, is that parents who have a child with an ASD may choose not to have any more children. This phenomenon is known as stoppage, and they say it may result in an underestimate of the recurrence risk.

The authors conclude the study by saying:

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.”

The results should be reassuring to parents who have a child with an ASD if they are thinking of having other children, note the researchers, because the recurrence risk they found is “substantially lower than recent reports from smaller clinic-based populations.”

Researchers from Duke University recently linked induced labor and autism risk.

How do parents navigate care for their children with disabilities and other needs? Here’s a detailed analysis: Getting support for children with disabilities.