Mothers who are obese during pregnancy have almost twice the odds of having a child with autism as women who weigh less, a U.S. study suggests.

When women are both obese and have diabetes, the autism risk for their child is at least quadrupled, researchers report in the journal Pediatrics.

"In terms of absolute risk, compared to common pediatric diseases such as obesity and asthma, the rate of autism spectrum disorder (ASD) in the U.S. population is relatively low, however, the personal, family and societal impact of ASD is enormous," said senior study author Dr. Xiaobin Wang, a public health and pediatrics researcher at Johns Hopkins University in Baltimore.

About one in 68 children have ASD, which includes autism as well as Asperger syndrome and other pervasive developmental disorders, according to the U.S. Centers for Disease Control and Prevention.

Put another way, that's about 1.5 percent of U.S. children. The study findings suggest the risk rises closer to about 3 percent of babies born to women who are obese or have diabetes, and approaches 5 percent to 6 percent when mothers have the combination of obesity and diabetes.

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To explore the connection between autism and maternal health, Wang and colleagues analyzed data on 2,734 mother-child pairs who were followed at Boston Medical Center between 1998 and 2014.

Most of the children, 64 percent, weren't diagnosed with ASD or any other development disorders, but there were 102 kids who did receive an ASD diagnosis.

Compared with typically developing kids, those with ASD were more likely to be boys, born preterm and at a low birth weight. Mothers of children with ASD were likely to be older, obese and to have diabetes diagnosed before or during pregnancy.

Maternal obesity was linked to a 92 percent increased risk for autism on its own, while diabetes diagnosed before pregnancy was associated with more than triple the risk.

When women both had diabetes and were obese, the autism risk compared to women with neither condition rose roughly four-fold if the diabetes was diagnosed during pregnancy and almost five-fold if diabetes was present before they conceived.

While the exact reason for these connections isn't clear, it's possible that increased inflammation, nutrients and hormones linked to diabetes and obesity may be responsible for the added autism risk, said Elinor Sullivan, a biology and neuroscience researcher at the University of Portland, in Oregon, who wasn't involved in the study.

"These factors impact how the brain develops," Sullivan said. "The risk for autism would be further increased if women were obese and had diabetes as the levels of inflammatory factors and nutrients that the offspring would be exposed to would be further elevated."

One limitation of the study is that some children with ASD may have been misclassified or had only tentative diagnoses, the authors note. In addition, it's possible that what's known as selection bias led more children with developmental delays to be included in the study.

The findings are important because more than a third of women of childbearing age are obese, roughly one in 10 have diabetes, and an estimated 2 percent to 10 percent of mothers develop diabetes during pregnancy, the authors note. Often, these conditions can be addressed prior to pregnancy.

"It is really good advice for women to achieve a healthy weight and get diabetes under control before becoming pregnant for many reasons not related to autism," Renee Gardner, a public health researcher at the Karolinska Institute in Solna, Sweden, who wasn't involved in the study said by email.

"The risks to both the child's health and the mother's health, including risk of serious pregnancy complications, C-section delivery and even still birth are greatly increased if the mother is obese or has uncontrolled diabetes," Gardner added.