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Largest-ever Autism Sibling Study Compares Risk between Full and Half Sibs

Shared risk higher among half sibs who share mom; findings underscore importance of monitoring baby sibs for early signs of autism
August 19, 2013


The largest-ever study on the recurrence of autism among siblings confirms a higher than typical risk among younger brothers and sisters of children with the disorder. It also documents increased risk for younger half siblings, but only when they share a mother. The two findings support previous research showing a strong inherited component to autism, as well as research suggesting that pregnancy conditions can play a significant role.

The study appears today in JAMA Pediatrics.

Danish researchers plumbed their national health registry for all children born from 1980 through 2004 – around 1.5 million. Within this group, they tracked the outcomes of all children with full or half siblings.

They determined that when an older brother or sister developed autism, a younger full sibling had a 7 percent chance (7 in 100) of developing it as well. This is significantly higher than autism’s prevalence in the general population (around 1.2 percent, or 1 in 88). However, it’s considerably lower than the 18 percent recurrence rate seen in previous autism sibling studies.

“Differences in how this study was conducted may explain the disparity with earlier findings,” says Michael Rosanoff, Autism Speaks associate director for public health research. The earlier, American studies looked at the siblings of children seen in clinics, while the Danish study looked at an entire population.

“This should not suggest that parents should be any less vigilant in monitoring the development of younger siblings of children who have autism,” adds Alycia Halladay, Autism Speaks senior director for environmental and clinical sciences. “Whether the risk is 7 percent or 18 percent, their risk is elevated,” she notes. “And we know that early intervention gives them the best chance at an optimal outcome.”

The ‘Mom factor’ among half siblings

Among half siblings, the Danish researchers found elevated risk only among younger brothers and sisters who shared the same mother as a child with autism. Around 2.4 percent of these younger siblings developed autism. That’s roughly double the rate seen in the general population. They found no significant increase in autism risk among younger half siblings who shared the same father.

"This suggests that, in addition to the genetic factors at play, there appears to be maternal influences such as effects during pregnancy,” Dr. Halladay comments. This supports a growing body of research on a range of pregnancy conditions that appear to increase or decrease the chances that a child will go on to develop autism.

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