(USA TODAY) -- In a study already being greeted with caution, Danish researchers report that children whose mothers had the flu or ran a fever lasting more than a week during pregnancy had an increased risk of developing an autism spectrum disorder.

U.S. health officials stress that the study, published in today's Pediatrics, is "exploratory."

The study analyzed data collected from 97,000 mothers of children born from 1997 through 2003. It found no association between mothers with colds, common respiratory or sinus infections, or urinary tract or genital infections during pregnancy and autism in their offspring.

Yet children whose mothers reported influenza during pregnancy had twice the risk of an autism diagnosis before age 3, and children whose mothers had a fever for more than seven days had a threefold risk.

There also was a small increased risk after the mother's use of various antibiotics during pregnancy.

"The study is really exploratory, and more research needs to be done," says Coleen Boyle, director of the Centers for Disease Control and Prevention's National Center on Birth Defects and Developmental Disabilities in Atlanta.

Autism researcher Irva Hertz-Picciotto, a professor at the University of California-Davis MIND Institute, who was not involved in the study, co-wrote a study in the Journal of Autism and Developmental Disorders that found fever during pregnancy more than doubled the risk of an autism or developmental delay in children. Flu during pregnancy was not associated with a greater risk.

"Mothers who reported a fever and reported not taking any medication to reduce fever were at higher risk to deliver a child that later developed autism. On the other hand, if they had a fever and took a medication to reduce fever, their child was not at higher risk," Hertz-Picciotto said in an e-mail.

Both her study and the new study raise the question of whether inflammation in response to infection may play some role in causing autism, Hertz-Picciotto says. She calls the new findings "noteworthy." Neither study proves causation, she says, "but add to the literature on fever and on infection."

For now, the standard clinical recommendations for treating pregnant women who have fever or flu should not change as a result of the findings, says Marshalyn Yeargin-Allsopp, chief of CDC's Developmental Disabilities Branch.

"We don't want women to not take antibiotics or not treat fever if they have the flu," she says.

Likewise, flu shots are crucial for pregnant women, says Denise Jamieson, chief of CDC's Women's Health and Fertility Branch.

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