A large study of Danish children — 96,736 of them — indicates that a bout of flu or fever lasting a week during pregnancy may be linked to increased risk of autism.

A large study of Danish children — 96,736 of them — indicates that a bout of flu or fever lasting a week during pregnancy may be linked to increased risk of autism. But the study, published in Pediatrics, also says the findings should be viewed cautiously until more research is done to confirm it.

Researchers from the University of Aarhus recruited the women and then called them multiple times over the course of the pregnancy and once after about new infections and medications they had taken.

A report in USA Today noted that U.S. health officials "stress that the study ... is exploratory and does not offer a specific cause of the developmental disability."

The study found that children whose mothers reported having influenza during pregnancy were twice as apt to be diagnosed with an infantile autism spectrum disorder, while children of women who reported fever lasting a week or more during pregnancy had threefold risk of being diagnosed with infantile autism.

Infantile autism is that diagnosed before age 3.

Even then, the numbers are still low: 99 percent of the women who had influenza or fever did not have a child who developed autism, lead author Dr. Hjordis Osk Atladottir told Reuters Health.

"I really want to emphasize that this is not something you should worry about," she said.

"Overall, we found little evidence that various types of mild common infectious diseases or febrile episodes during pregnancy were associated with ASD/infantile autism," the study authors wrote.

Other infections like urinary tract, the common cold, sinus infections and genital infections were not linked to higher rates of autism in the retrospective study of the children, who were born between 1997 and 2003. They did find a small increase in the incidence of autism spectrum disorder among women who used antibiotics.

Autism spectrum disorders vary greatly from person to person across a range that can be very mild to severe. The developmental disabilities can cause "significant social, communication and behavioral challenges. People with ASDs handle information in their brain differently than other people," according to the CDC fact sheet on the disorder. Types include classic autism, atypical autism and Asperger's syndrome.

"This study is really exploratory and more research needs to be done to understand how maternal infections, as well as other risk factors, influence the risk of autism spectrum disorders," Coleen Boyle, director of the Centers for Disease Control and Prevention's National Center on Brain Defects and Developmental Disabilities, told USA Today. "We need to have more information to get a better sense of what's going on here."

Autism researcher Irva Hertz-Picciotto, professor of environmental epidemiology at University of California Davis MIND Institute, who was not part of the study, told the newspaper that the findings were nonetheless "noteworthy" because so many mothers were interviewed during and shortly after pregnancy, when it was not known whether the child would have an autism spectrum disorder. That eliminated "recall bias," she said.

Her own earlier research had found that fever in pregnancy doubled the risk of autism or a developmental delay, but did not see an association with influenza and increased risk.

U.S. health officials emphasized that there are many reasons to treat fever and flu during pregnancy, including links to other ills, so this finding should not change how fever or flu is treated during pregnancy.

Atladottir told Reuters that some research in rodents suggests that women's activated immune cells can cross the placenta and change chemicals in a baby's brain before birth. How or if it applies to humans is unknown.

Animal studies "suggest that maternal immune infection produces long-lasting changes in the brain, including those seen in individuals on the autism spectrum," Alycia Halladay, senior director of environmental and clinical sciences at Autism Speaks, told HealthDay reporter Amanda Gardner. "Research suggests specific chemicals, called cytokines, may mediate this effect."

Cytokines, Gardner wrote, are message-carrying cells of the immune system. Atladottir speculated to her that since some cytokines can cross the placental barrier, in this cause they might be "able to alter the release of neurotransmitters and thus affect fetal brain development."

The researchers also pointed out that doctors did not confirm their reports of flu and it's possible that influenza could be mistaken for something else and vice versa.

Regardless, "It is highly recommended that women avoid infection during pregnancy, and there are a variety of very practical ways to decrease the likelihood of this," Paul Patterson, who studies the immune system and brain development at the California Institute of Technology, told Reuters.

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