Older mothers are far more likely to have autistic children than those who give birth younger, say researchers who have examined data on every birth in California in the 1990s.

The team found that mothers over 40 were 51% more likely to have an autistic child than mothers between 25 and 29 (see table for absolute risks).

“This study challenges a current theory in autism epidemiology that identifies the father’s age as a key factor in increasing the risk of having a child with autism,” says study author Janie Shelton, of the University of California, Davis (press release).

What Shelton and her colleagues found is that maternal age consistently increases autism risk but paternal age is only important when the father is older and the mother is under 30. The reason older parents experience these problems is not clear, although genetic, endocrine, immunological and environmental explanations have all been suggested.

“We still need to figure out what it is about older parents that puts their children at greater risk for autism and other adverse outcomes, so that we can begin to design interventions,” paper author Irva Hertz-Picciotto told AFP.

Recent trends for delaying childbearing led to a 4.6% increase in autism diagnoses in California over the 1990s, the researchers say. However the overall increase in autism diagnoses is greater than this, and isn’t just confined to older mothers.

“I don’t think a mom blaming herself is going to help us understand what’s causing autism or help prevent further cases,” says Shelton (CNN). “I would urge parents not to blame themselves, regardless of what age they are.”

The study is published in Autism Research.

Table: UC Regents