The study was based on a survey of mothers of nearly 97 000 children aged eight to 14 and born between 1997 and 2003 in Denmark. Only 1% (976) of the children were diagnosed with autism.
Researchers said that mild and common respiratory, sinus, urinary tract or genital infections, as well as feverish episodes or use of antibiotics in mothers during pregnancy were not a strong risk factor for autism.
But children whose mothers reported experiencing a bout of influenza during pregnancy had twice the risk of being diagnosed with autism, while those whose mothers had a fever lasting more than seven days before gestational week 32 were three times more likely to be diagnosed with the disorder.
Pregnant women have a weakened immune system during the nine months of gestation.
The study, published in the US journal Pediatrics, also noted a small increase in the risk of autism after the mother used different antibiotics during the pregnancy.
However, the researchers stressed that "we do not know whether the antibiotic treatment itself caused the observed association or whether the antibiotic use functioned as a proxy variable for an underlying disease."
They also stressed that the link between fever during pregnancy and autism could be a "coincidental finding," and required further study.
Given the link found between autism and fever or the flu during pregnancy, the study's authors recommended that pregnant women get a flu shot.
(Sapa, November 2012)