Amid a range of possible causes for autism spectrum disorder (ASD), including maternal obesity, the environment, and abnormal gut bacteria, a new study finds that a father’s excessive weight may put his child at risk for the disorder as well.

“We were very surprised by these findings because we expected that maternal obesity would be the main risk factor for the development of ASD,” said Dr. Pal Suren, of the Norwegian Institute of Public Health, in a statement. “It means that we have had too much focus on the mother and too little on the father. This probably reflects the fact that we have given greater focus to conditions in pregnancy, such as the growth environment for the fetus in the womb, than both environmental and genetic factors before conception.”

The findings provide further evidence that ASD is inherited rather than developed after birth. The researchers found that a child’s risk of ASD, which includes Asperger’s syndrome, increased with the body mass index (BMI) of their fathers. Among children over 7 years old with obese fathers, the risk of autism was double that of children who were born to fathers of a normal weight, MedPage Today reported. The researchers were surprised by the findings because they were unable to find a link between maternal weight and autism, despite previous evidence from other studies.

They looked at data from the Norwegian Mother and Child Cohort Study, which contained information from over 90,000 children at 3, 5, and 7 years old. The study looked at the physical and mental health of both mothers and fathers — they answered these questions referring to when their partners were pregnant — as well as their children. They then compared this data to that of the Norwegian Patient Registry, which held information about children who were referred for evaluation and treatment of possible ASD.

Yet, even though they found a link between obese fathers and children with ASD, the link was too small to say obesity was a cause. The risk of a child with an obese father developing ASD was only 0.27 percent, compared to 0.14 percent in children with normal-weight fathers. Regardless, the study is a step in a new direction for autism research. “We have a long way to go,” Suren said in the statement. “We must study genetic factors in the relationship between obesity and autism, as well as environmental factors associated with switching the genes on or off — so-called epigenetic factors.”  

Source: Suren P, Gunnes N, Roth C, et al. Parental Obesity and Risk of Autism Spectrum Disorder. Pediatrics. 2014.