The causes of autism spectrum disorder are still inconclusive, however, research has found that vaccines, parental age, complications during pregnancy, and genetics could all have a role in its development. Now, a new study brings more genealogical evidence, saying that children who have older siblings with autism are seven times more at risk of also developing the disorder.

Autism Spectrum Disorder and Genetics 

Researchers at Aarhus University in Denmark gathered data from almost 1.5 million children who were born in Denmark between 1980 and 2004. They followed up with all the families at the end of 2010. They found that the overall rate of autism diagnoses was slightly above one percent, which was consistent with previous studies. Among children with older siblings with autism, the rate rose to 6.9 percent, according to the LA Times.

The rate of autism among children who had older maternal half-siblings with the disorder was also higher than the average, at 2.4 percent, whereas the link between half-siblings with the same father showed no significant increase. This evidence suggests that there could be more of a maternal link between autism and genetics, or at least pregnancy complications, because a study from earlier this month found that children born to mothers whose pregnancies were either induced or augmented had a 27 percent higher chance of developing the condition.

The rate of autism diagnoses in this study was slightly lower than the rates found in previous studies. This was because it compared recurrence rates to actual rates of autism, rather than estimated rates of autism within the population. Also, because the rate was higher in maternal half-siblings, and other research has shown evidence that maternal antibodies during pregnancy could cause the disorder, study leader, Therese Gronberg, believes that the causes are much more complex than just genetics. 

“There’s definitely genetics going on in the pathway to autism spectrum disorders, but what you also see is that something else is going on as well,” Gronberg told the Times. “Compared to what previously was believed, it supports a lower genetic role.”  

“If it was only genetics we would see a much higher recurrence rate among siblings,” she told Reuters.

Autism spectrum disorder includes a wide range of symptoms, however, many cases fall into three categories: social impairment, communication difficulties, and repetitive and stereotyped behavior. In some cases, the disorder doesn’t develop until their second or third year of life, during which time they begin to lose interest in others, become silent, withdrawn, or indifferent to social signals. When babies develop the disorder, they oftentimes become too focused on particular objects, rarely make eye contact, and avoid engaging in play with their parents. One in 88 children develop the disorder and boys are almost five times more likely to have it, according to the Centers for Disease Control and Prevention (CDC).

Taking Steps To Prevent Autism Spectrum Disorder

Pregnant mothers can lower their child’s risk of autism by taking prenatal folic acid and avoiding exposure to toxins, according to Time. Taking steps during a child’s first few months of life can also help to prevent the onset of the disorder.

“We know that early intervention can make a real lifetime of difference,” Alycia Halladay, senior director of environmental and clinical sciences for Autism Speaks, told Time. “So be very vigilant during that child’s life, all the way from birth to the well-baby check-ups, [at] six months, 12 months, and 18 months. Make sure you are watching for the signs and symptoms of autism. Consult your pediatrician, and if you do notice the signs and symptoms of autism you can receive help free of charge from a state-based early intervention agency.”

Source: Gronberg T, Schendel D, Parner E. Recurrence of Autism Spectrum Disorders in Full- and Half-Siblings and Trends Over Time: A Population-Based Cohort Study. JAMA Pediatrics. 2013.