SALT LAKE CITY — In the last ten years, rates of children born with autism have started to increase, creeping from roughly 1 in 150 kids in 2000, to 1 in 88 in 2008. In Utah, that number is much higher — roughly 1 in 47. Scientists and doctors aren't entirely sure why these numbers continue to rise.
A massive study of almost 97,000 children may have a few answers. Researchers from both Denmark and California think the flu that plagues folks every winter with chills and fevers may have something to do with it.
During the study, which followed roughly 30 percent of the child-bearing women in Denmark from 1997 to 2003, researchers found that women who had the flu during pregnancy were about twice as likely to have children with autism as those who did not have the flu.
For those who were especially sick, the risk was even greater: A flu or fever that lasted for a week led to a tripled risk of autism.
"We know that genes are important," Dr. William McMahon with University of Utah Health Care told KSL. "We think that environmental triggers are important. This study is a nice hint."
Inflammation due to the mother's immune response is implicated in the results.
"Growing evidence suggests inflammatory processes may be interfering with brain development at critical stages, leading to changes in behaviors such as those associated with autism, as well as cognitive deficits," Dr. Irva Hertz-Picciotto of Stanford University's told NBC News.
This connection has been proposed for some time. Studies among animals have shown that the mother's immune response causes changes in the brain of offspring. But vaccines don't cause the same immune responses as a full-blown infection or a long-term sickness.
"We want to reassure women. In this study, most women who experienced flu or prolonged fever or who were taking antibiotics did not have children with an autism spectrum disorder," Dr. Coleen Boyle told NBC News.
We shouldn't fear too much, however — 99 percent of women who had the flu did not give birth to children with autism. Researchers also stressed that the results are not set in stone. Flu could easily have been over-reported or misreported. The study is exploratory and does not necessarily have implications for treatment or diagnosis yet.
"If you look at the numbers, it's barely significant in a rigorous statistical way of testing," McMahon said.
Nevertheless, pregnant mothers are encouraged to get flu shots for this and many other reasons.
"All of us should have flu shots. Women contemplating getting pregnant or even pregnant women, unless they have some other illness that precludes it, should have a flu shot," McMahon said.
Contributing: Nadine Wimmer