NEW YORK (Reuters Health) - Advanced maternal and paternal age are both independent risk factors for having a child with an autism spectrum disorder (ASD), the results of a large birth cohort study suggest. However, the researchers emphasize that even the oldest parents have a very small risk of having a child with an ASD.

"A number of studies have looked at the impact of maternal age on autism, but the results have been inconsistent. Paternal age has been much less studied," lead author Dr. Lisa A. Croen, from Kaiser Permanente in Oakland, California, told Reuters Health. "Our findings support a link between advanced (parental) age and autism, although the effect on risk is small."

According to Dr. Croen, the new findings do not make a strong argument for or against having children at a younger age.

The study, which is reported in Archives of Pediatrics and Adolescent Medicine for April, involved all singleton children who were born at Kaiser Permanente between 1995 and 1999. Of the children studied, 593 were later diagnosed with an ASD (follow-up through May 2005) and 132,251 were not.

With each 10-year increase in maternal and paternal age, the ASD risk rose by 31% and 28%, respectively. Compared with parents between 25 and 29 years of age, mothers and fathers who were 40 years or older were 27% and 52% more likely, respectively, to have a child with an ASD.

For autistic disorder, each 10-year rise in maternal and paternal age translated into an 18% and 34% increase in risk, respectively. The corresponding incremental risks for pervasive development disorder not otherwise specified or Asperger disorder were 45% and 24%.

The link between advanced parental age and ASD risk was stronger for girls than for boys, but the association was not statistically significant.

Dr. Croen said that she was not surprised by the findings, given previous reports supporting a link and the fact that a number of childhood diseases are associated with advanced parental age. She added that her group has now explored this topic further using data from the entire state of California and plans to submit these findings shortly for publication.