A mother’s fever during pregnancy, especially in the second trimester, is associated with a higher risk that her child will be diagnosed with autism spectrum disorder, researchers reported Tuesday. Three or more fevers after 12 weeks of gestation may be linked to an even greater risk of the condition.

The study by researchers at Columbia University’s Mailman School of Public Health adds support for the theory that infectious agents that trigger a pregnant woman’s immune response may disrupt a fetus’s brain development and lead to disorders such as autism.

“Fever seems to be the driving force here,” not the infection itself, said Mady Hornig, director of translational research at the school’s Center for Infection and Immunity. Fever can be part of the body’s immune response to an infection, and molecules produced by a mother’s immune system may be crossing into the baby’s neurological system at a critical time, she said.

The research, published in the journal Molecular Psychiatry, comes at a time when the scientifically discredited theory that some childhood vaccines cause autism has gained new attention. President Trump has promoted this myth, energizing some anti-vaccine groups.

Some families say that their children developed autism after vaccinations. The timing is a coincidence, however; symptoms of autism typically become clear at around 2 years of age, which happens to be the age when children get certain vaccines.

The study of fever during pregnancy adds to evidence that genetic and environmental factors influence the risk of autism well before children start showing symptoms. Other research has tied vitamin D deficiencies during pregnancy to an increased risk of autism, and brain activity in 6-month-olds can help predict their likelihood of being diagnosed at 2.

Other research has found a link between a father’s age at conception and a child’s chance of developing autism.

The new study looked at 95,754 Norwegian children born between 1999 and 2009, identifying 15,701 children whose mothers said they had fevers at some point in their pregnancies. Of those, 583 had children later diagnosed with autism spectrum disorder.

The information was gathered during the pregnancies and shortly after birth, so the mothers’ “recall bias” was minimized, Hornig said.

Women who had one or two fevers during their pregnancies were 1.3 times more likely to have a child eventually diagnosed with autism spectrum disorder, the study found. Three fevers doubled the risk, although the number of cases was too small to be considered statistically significant, according to the research.

Acetaminophen, the drug most commonly taken to reduce fever, had no significant impact on the rate of autism spectrum disorder, the researchers found, though the number of women who reported using the medication was small.

Hornig said additional research into  inflammation and pregnancy is needed. “What I want to know is the mechanism,” she said. “I want to understand how this inflammation may relate” to autism spectrum disorder.