Autism Risk & Half-Siblings


My child from a previous marriage has autism. I’m now remarried and wondering what the chances are that we will have a child affected by ASD?

This week’s “Got Questions?” answer is from Autism Speaks Director of Research for Environmental Sciences Alycia Halladay, Ph.D.


Last August, a well-designed study established that the risk of autism for a sibling of a child with autism is about 19 percent. Since that study came out, we have received many questions from mothers and

fathers who have a child with autism but are now in a new relationship. Like you, they are concerned about the risk for autism for children with their new partner – half-siblings to the child with autism.

Until now, we haven’t been able to provide accurate risk estimates for half-siblings. But a new study published online this month in the journal Molecular Psychiatry addressed this question. The study found that half-siblings of those with autism are likewise at increased risk for autism spectrum disorder (ASD). This reinforces previous findings on the strong influence of genetics on the development of ASD. However, it does not identify the genes involved nor explain how these genes may be interacting with nongenetic, or environmental, influences to increase risk.

The study included 5,237 families with an ASD-affected child and at least one sibling. Among these families, 619 included at least one maternal half-sibling and 55 included a paternal half-sibling. This study used the database of the Interactive Autism Network, which is partly funded by Autism Speaks and includes a cross-section of autism families.

The researchers found that autism risk for maternal half-siblings in the study was about half of the rate of full siblings – or 5 to 7 percent versus 10 to 11 percent. In paternal half-siblings, the risk was not significantly increased compared to the general population. However, the number of paternal half-siblings available for the analysis was too small to conclude that there wasn’t an increased risk on the paternal side. However, previous studies have clearly shown increased risk with older dads and even shed light on possible genetic mechanisms. (See our related news story here.)

You might be wondering why this study found a lower concordance rate of 10 to 11 percent among full siblings when last year’s sibling’s study came in at 19 percent. These two studies used different designs, which affected their results. However, their differences don’t change the bottom line: Any biological sibling of someone with autism – either a full sibling or half – has a higher than average risk of developing an ASD.

What does this mean for parents? If you are considering getting pregnant, talk to your obstetrician about what you can do to have the healthiest pregnancy possible.  In particular, I would encourage you to discuss how you can avoid environmental risks such as those I discussed in my earlier blog, Avoiding Environmental Hazards During Pregnancy. These recommendations apply to all pregnant women, not just those who have a child affected by ASD.

After the birth of your child, talk to your pediatrician about your baby’s risk and the need for close monitoring of developmental milestones. (See Learn the Signs on our website.) Don’t hesitate to voice any concerns you have about your child’s development and request screenings if you or your healthcare provider feels they may be needed. The American Academy of Pediatrics has long recommended that all children be screened for autism at their 18 month well baby checkups. Your baby’s doctor can also use this validated baby-toddler checklist as early as 12 months. It is a useful initial screen for not only autism but also other developmental delays and disorders.

Please also see How Is Autism Diagnosed? and What Are the Symptoms of Autism? By monitoring your infant closely and beginning early intervention if necessary, your child will have the best possible outcome.

Meanwhile, Autism Speaks will continue to fund research on the early detection and treatment of ASD through a number of programs and initiatives including our High-Risk Baby Siblings Consortium and Toddler Treatment Network. Thanks to our families, donors and other supporters for making this research possible.

Got more questions? Please send them to and join our monthly “The Doctors Are In” webchats, the first Thursday of every month at 3 pm Eastern, via the Live Chat tab on our Facebook page.


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Autism Speaks
Autism Speaks has grown into one of the world's leading autism science and advocacy organizations. Visit for more information.
Autism Speaks

Autism Speaks

Autism Speaks has grown into one of the world's leading autism science and advocacy organizations. Visit for more information.

0 thoughts on “Autism Risk & Half-Siblings

  • February 28, 2013 at 6:23 am

    Thankyou. Very good advice. Also thankyou for the research and efforts into helping parents understand. It’s a daunting road especially at first when you don’t see the rainbow at the end of the diagnoses.

    Just had my 2nd child who although is not a half sibling, I wondered throughout my pregnancy was I going to deliver another ASD child.  Actually this weighted heavily upon my mind too much, I had to try to relax. Especially (even though you did not mention the stats) I had another boy. He is now 11 months and yes still monitoring his progress closely.

    My advice, yes give everything you can when you are pregnant to give the best possible chance for your unborn. Yes, milestones are very important too.

    IMO the best advice is to don’t worry. I believe worry and anxiety causes more harm than good to you both, pregnant and raising them.

    No stats on my advice but it’s hard enough to be pregnant and raise a new born. Don’t complicate it with worry. Enjoy it because you are going to love them the same, if not more. Don’t make your stress their problem. They are perfect no matter what the world believes to be true.


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