Mothers of 15,701 (or 16 percent) of the children reported having a fever (a symptom of infection) at least once during their pregnancy.
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In the U.S., 20 percent of women report one or more fever episodes, according to the study.
Among those mothers, researchers identified 583 cases of autism spectrum disorder in the children.
Here’s what the researchers found:
- The risk of autism increased by 34 percent when mothers reported a fever over 99 degrees at any time during pregnancy.
- The risk of autism increased by 40 percent when fevers occurred in the second trimester.
- The risk of autism increased by more than 300 percent when women reported having three or more fevers after the twelfth week of pregnancy.
- Researchers could not definitively say how women who took anti-fever drugs like acetaminophen/Tylenol or ibuprofen/Advil might be affected by ASD risk due to low numbers.
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The findings support the idea that infection during pregnancy (and how the immune system responds to it) could play a role in the development of some cases of ASD, according to the researchers.
"Future work should focus on identifying and preventing prenatal infections and inflammatory responses that may contribute to autism spectrum disorder," senior author W. Ian Lipkin said in a statement.
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While previous research has documented the link between fever during pregnancy and autism, the Molecular Psychiatry study is the most robust to date.
Read the full study.