MONDAY, May 9 (HealthDay News) -- Children conceived in winter seem to have a greater risk of being diagnosed with autism, a new study suggests.

Environmental factors -- including exposure to seasonal viruses such as influenza and changes in diet -- may play a role in the greater risk for autism among children conceived during the winter, according to the University of California, Davis researchers.

The investigators analyzed data from 6.6 million children who were born in California between January 1990 and December 2002 and followed up until the children were 6 years old. The risk of an autism diagnosis was higher for children conceived in December, January, February and March than for those conceived in other months of the year, the study found.

Compared with children conceived in July, the risk for autism was 8 percent higher among those conceived in December and 16 percent higher for those conceived in March, according to the report, published online May 3 in Epidemiology.

"Studies of seasonal variations can provide clues about some of the underlying causes of autism," Irva Hertz-Picciotto, chief of the division of environmental and occupational health at the University of California-Davis School of Medicine, said in a university news release. "Based on this study, it may be fruitful to pursue exposures that show similar seasonal patterns, such as infections and mild nutritional deficiencies."

"However, it might be that conception is not the time of susceptibility," Hertz-Picciotto added. "Rather, it could, for instance, be an exposure in the third month of pregnancy, or the second trimester, that is harmful. If so, we might need to look for exposures occurring a few months after conceptions that are at higher risk -- for example, allergens that peak in the spring and early summer."

More information

The U.S. National Institute of Neurological Disorders and Stroke has more about autism.