The children of obese fathers may be at a 53 percent higher risk of autism than children whose fathers are a healthy weight, reports a large Norwegian study published 7 April in Pediatrics.

Other studies have linked mothers’ obesity to autism risk in their children, but they didn’t factor in the fathers’ weight. This may have led to an overestimation of the mother’s role in the risk of autism, the researchers say.

The new study dropped the autism risk associated with having a heavy mother from 17 percent to 9 percent after taking the father’s weight into account.

The researchers examined the medical records of nearly 93,000 children, including 419 diagnosed with autism spectrum disorders, in the Norwegian Mother and Child Cohort Study. In this study, researchers recruited pregnant women between 1999 and 2008 and followed their families through early childhood.

The new study calculated the parents’ body mass index (BMI), a measure of body fat based on weight and height, using questionnaires given to the children’s mothers at 18 weeks of pregnancy. About 10 percent of mothers and fathers were obese, with a BMI of 30 or higher. People with healthy weights have a BMI between 18 and 25.

The study examined risk for subtypes of autism, including pervasive developmental disorder-not otherwise specified (PDD-NOS) and Asperger syndrome, which are considered mild forms of the disorder. Both are now part of the autism diagnosis in the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders, the American Psychiatric Association’s latest guidelines for diagnosis.

The risk related to obesity varies among these subtypes, the study found. Obese men have a 73 percent increased risk and obese women have a 34 percent increased risk of having a child with classic autism compared with parents who are at healthy weights. For Asperger syndrome, obese men are at double the risk and obese women are at a 40 percent increased risk.

As the father’s weight goes up, so does the risk of autism and Asperger syndrome, the study found. Obesity in either parent is not associated with an increased risk of PDD-NOS.  

The researchers also examined medical and lifestyle factors in the parents’ lives that may have affected their children’s autism risk. According to data from the study, obese mothers and fathers are less educated and smoke more than do parents with a healthy BMI.

Obese mothers are also less likely to have taken folic acid supplements before pregnancy than women of a healthy weight. Women who take folic acid supplements around the time of conception are less likely to have a child with autism, research shows.

Heavy women in the study also have a higher risk of preeclampsia — a condition characterized by high blood pressure during pregnancy that can lead to seizures — as well as premature delivery. They are also more likely to have type 2 diabetes and gestational diabetes, which studies have shown may increase the risk of autism in the child.

Overall, however, it’s unclear why a father’s weight might increase his child’s autism risk. Genetics may play a role, the researchers say. For instance, deletions on chromosome 16p11.2 are implicated in both autism and morbid obesity, and fathers may pass these genetic variants on to their children.