Researchers have found a clear correlation between the month a child was conceived and their risk of being diagnosed with autism.

The team from the School of Medicine at the University of California, Davis, reviewed the birth records of more than 7 million children born in the 1990s and early 2000s.

They found those children in the study who were conceived in winter had a significantly higher risk of being diagnosed with autism.

"The risk of having a child with an autism spectrum disorder grew progressively throughout the autumn and winter to early spring, with children conceived in March having a 16 per cent greater risk of later autism diagnoses, when compared with July conceptions," the researchers said.

They admit they do not know for certain why seasons are important.

"The finding suggests that environmental factors, for example, exposure to seasonal viruses like influenza, might play a role in the greater risk they found of children conceived during the winter having autism," they said.

One of the strong points in this research is the scope of the study.

Researchers reviewed 7.2 million birth records from California and were able to follow 90 per cent of the children until they turned six to see if they developed autism.

Other smaller studies into autism risk and conception or birth have had mixed results.

None had the large numbers of children found in the Californian study.

"Studies of seasonal variations can provide clues about some of the underlying causes of autism," said Irva Hertz-Picciotto, chief of the division of environmental and occupational health in the Department of Public Health Sciences in the UC Davis School of Medicine.

"Based on this study, it may be fruitful to pursue exposures that show similar seasonal patterns, such as infections and mild nutritional deficiencies."

The study is published online today in the journal Epidemiology.