Children whose moms have any type of fever during pregnancy may have slightly increased odds of developing an autism spectrum disorder, a new study suggests.
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The large study found that one episode of fever in the second trimester might increase the risk for autism by 40 percent.
Several bouts of fever after the twelfth week of pregnancy could raise the risk threefold, researchers reported.
"Fever is a response to a wide range of infections, and it is common during pregnancy," said lead researcher Dr. Mady Hornig.
She's an associate professor of epidemiology at Columbia University's Mailman School of Public Health in New York City.
But she pointed out, "The absolute risk is low. The vast majority of women who get an infection with fever, even flu, are not going to end up having a child with autism."
Hornig also cautioned this study cannot prove that a fever during pregnancy causes autism, only that there appears to be an association.
The researchers also cannot tell what it is about fever that might have this effect.
Hornig speculated that it might involve the body's reaction to a fever-causing infection that also has an effect on fetal brain development.
"There is something in the mother's immune response that may increase the risk for the infant," Hornig said.
"But it's not in every mother. We don't think this is a pathway for autism. We don't think it's the only way autism is triggered in children."
Surprisingly, the researchers didn't see much reduction in the fever-associated risk when women took acetaminophen (Tylenol), Hornig said. "We anticipated that we would have seen more of an effect," she said.
Thomas Frazier, the chief science officer of Autism Speaks, said, "Unfortunately, the study is not able to determine exactly what is driving this relationship - the fever itself, any maternal infection, or specific types of infections."
For the study, Hornig and colleagues collected data on nearly 100,000 children born in Norway between 1999 and 2009.
Among these kids, nearly 600 were diagnosed with autism spectrum disorder.
Mothers of almost 16,000 children reported having one or more fevers during pregnancy.
That rate - 16 percent - is similar to the rate of fevers reported by pregnant women in the United States, Hornig said.
The researchers found that the risk for autism was increased 34 percent when mothers reported fever at any time during pregnancy. ■