According to a ten-year exhaustive study of all births in the state of California during the 1990’s, UC Davis Health System researchers have found that advanced maternal age, defined as pregnancy after the age of 35, is linked to a significantly greater risk of having a child with autism.
During the 1990’s, the number of California women over 40 giving birth increased by more than 300 percent. Researchers gathered information on all California births between January 1, 1990 and December 31, 1999 and identified those children with a diagnosis of full-syndrome autism through a California Regional Center, a part of the state’s department of Developmental Services. The total number of cases in the study sample was approximately 4.9 million births with 12,159 documented cases of autism.
The study found that for each five-year increase in a mother’s age, the risk of having an autistic child increased incrementally by 18%. A 40-year-old woman’s risk of having a child diagnosed with autism was 50% greater than that of a woman between 25 and 29 years of age.
Advanced paternal age was only found to be a risk factor when the mother was under the age of 30. For example, a child born to a mother under the age of 25 and fathered by a man over the age of 40 was twice as likely to develop autism than those whose father was between 25 and 29 years old.
The researchers note that understanding the relationship between increased parental age and autism risk is critical to the understanding of the biological cause. One theory on the link was found in a 2008 UC Davis study that found that some mothers of children with autism had antibodies to fetal brain protein. Advancing age was associated with this increase in autoantibody production. Other theories include the accumulation of environmental chemicals over time and chromosomal defects known to be at higher risk for children born of older mothers.
Geraldine Dawson, chief science officer for Autism Speaks, praised researchers for their study of parental age, but urged older mothers not to worry unnecessarily. “We need to look at this finding in context,” she said. “This study is important, but we are not going to find one factor that can explain this dramatic increase in the prevalence of autism. We are going to find multiple factors, and advanced maternal age appears to be one of them.”
Autism is a developmental disorder that causes deficits in social skills and communication, as well as repetitive and restricted behaviors. It is estimated to occur in 1 to 100 children in the United States with onset prior to the age of 3. Abnormal brain development is known to be fundamental to the behaviors that characterize autism.
The research is published online in the February issue of the journal Autism Research.