The older the mother the greater the risk of having a child with autism, regardless of the father's age, according to a study of all births in California during the 1990s by UC Davis Health System researchers. The study is one of the largest population-based studies to quantify how each parent's age — separately and together — affects the risk of having a child with autism. The study also found that a father’s advanced age is associated with an elevated autism risk only when the father is older and the mother is under 30.

The findings, published online in the journal Autism Research, found that the incremental risk of having a child with autism increased by 18 percent — nearly one fifth — for every five-year increase in the mother's age. A 40-year-old woman's risk of having a child later diagnosed with autism was 50 percent greater than that of a woman between 25 and 29 years old.

 

Though a father’s advanced age is a known risk factor for having a child with autism, previous research has shown contradictory results regarding whether it is the mother, the father or both who contribute most to the increased risk of autism.

 

“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 Janie Shelton, the study's lead author and a doctoral student in the UC Davis Department of Public Health Sciences. “It shows that while maternal age consistently increases the risk of autism, the father's age only contributes an increased risk when the father is older and the mother is under 30 years old. Among mothers over 30, increases in the father's age do not appear to further increase the risk of autism.”

 

During the 1990s, the number of California women over 40 giving birth increased by more than 300 percent. But only about 5 percent of the 600-percent increase in the number of autism cases in the state can be attributed to women waiting longer to have children, the study suggests.

 

Irva Hertz-Picciotto, professor of public health sciences, a researcher at the UC Davis MIND Institute and the study's senior author, say the reason that having an older parent places a child at risk for autism is not known.

“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,” Hertz-Picciotto says.

One possible clue comes from a 2008 UC Davis study that found some mothers of children with autism had antibodies to fetal brain protein, while none of the mothers of typical children did. Advancing age has been associated with an increase in autoantibody production. Further work investigating advancing age in such findings may be useful, the study authors say. They added that some persistent environmental chemicals accumulate in the body and also may have a role to play in autism, possibly contributing to the apparent effect of parental age.