WEDNESDAY, May 11 (HealthDay News) -- Having the flu during pregnancy isn't associated with a heightened risk of autism or developmental delay in children, although having a fever during pregnancy might be.

And giving birth by Cesarean section isn't associated with autism in offspring, but having diabetes or high blood pressure or being obese while pregnant seems to be.

Those are the findings of several new studies that sought to uncover what factors during pregnancy may have an impact on the risk of autism in children. The research was presented Wednesday during a press conference at the International Meeting for Autism Research (IMFAR) in San Diego.

About one in 110 U.S. children have an autism spectrum disorder, a neurodevelopmental condition characterized by problems with language, communication and social functioning, as well as repetitive or restrictive behaviors. Many children with autism also have other conditions, including epilepsy, anxiety disorders and gastrointestinal problems.

Recent research has found that the diagnoses of autism is on the rise, some of which might be attributable to greater awareness and increased availability of autism services, experts said. Still other research suggests the actual incidence of autism is rising as well.

"Autism is an incredibly complex disorder than now affects nearly 1 percent of children," said IMFAR President David Amaral.

The cause of autism is unknown, although some research, including the papers presented at IMFAR, hint at some maternal factors that might contribute to a child developing the disorder.

In the first study, researchers looked at data from research on 1,000 kids taking part in the population-based Childhood Autism Risks from Genetics and Environment (CHARGE) Study, which included 462 with an autism spectrum disorder, 136 with other development disorders and 265 typically developing kids between 2 and 5. The researchers also examined mother's reports of influenza or fever during pregnancy.

Researchers found no link between autism or other developmental delay and the flu during any trimester of pregnancy.

But in a weighted analysis, the mothers of children who were diagnosed with an autism spectrum disorder were nearly twice as likely to report having a fever during pregnancy than mothers of kids without autism, said study co-author Irva Hertz-Picciotto, deputy director of the University of California, Davis's MIND Institute (Medical Investigations of Neurodevelopmental Disorders).

The risks were particularly elevated when mothers reported having a fever during the 2nd trimester.

Prior research has suggested an association between viral infections during pregnancy and autism. Yet researchers believe the mother's inflammatory response to the virus, rather than any particular virus itself, influences the child's brain development.

"These results add to a growing body of evidence that maternally mediated inflammation might be part of the mechanistic pathway leading to autism," Hertz-Picciotto said.

The researchers stressed, however, that the findings were preliminary and that the association did not reach statistical significance in an analysis where they did not apply the sampling weights.

Along that same vein, another study to be presented at the meeting by researchers from Denmark analyzed samples of amniotic fluid collected from pregnant women in Copenhagen since 1982. Some of the women's children were later diagnosed with autism.

The study found no differences in hospitalizations for infections during pregnancy, but they did find that the mothers of autistic kids were more likely to have elevated levels of certain inflammatory markers -- specifically, a cytokine called TNFa. For reasons not well understood, the inflammatory markers were particularly high in the mothers of girls with autism.

"Overall, findings from our study not only support the idea that immune dysfunction plays an important role in autism, but also confirm that this is happening actually before birth," the authors wrote.

In a third study, which also drew from data from the CHARGE Study, researchers looked at mothers who had diabetes (Type 2 and gestational), chronic hypertension and pre-pregnancy obesity.

The study found that the children of mothers with those conditions were significantly more likely to have autism or another developmental delay.

"For mothers with at least one of the three conditions, their children had a 60 percent increased risk of autism and for developmental delays, there was a 150 percent increased risk," Hertz-Picciotto said.

Researchers also noted that children of diabetic mothers also tended to have poorer expressive language skills whether the kids had autism or not.

A fourth study, which looked at Cesareans and autism, also used data on about 500 kids with autism and 300 without autism from the CHARGE study.

The study found no link between birthing method and autism.

However, having an emergency C-section after a difficult labor -- including prolonged labor, membrane rupture or infection of the amniotic cavity -- was associated with a heightened autism risk in offspring.

Yet when researchers controlled for other factors, such as maternal age, pre-eclampsia and diabetes, that are associated with heightened autism risk, the mode of delivery was no longer statistically significant.

The good news is that many of the factors described in the three studies are "modifiable," meaning that if they're confirmed to be risk factors for autism, it may be possible to alter them, Hertz-Picciotto said.

"We'd like to be able to intervene and potentially prevent autism in the future," she said.

Experts note that research presented at meetings has not been subjected to the same type of rigorous scrutiny given to research published in peer-reviewed medical journals and should be considered preliminary.

More information

The National Institute of Neurological Disorders and Stroke has more on autism.