Autism is a cognitive disorder characterized by impaired social interaction and communication, and by restricted and repetitive behavior. These signs start appearing before a child is three years old.

Study details and findings
To reach this conclusion, a research team from the University of California Davis, U.S., examined birth records in California from 1990 to 1999.

Over the study period, there were about five million births, and more than 12,159 autism cases.

Based on the data, the researchers found that women aged 40 or above had 50 percent elevated chances of having an autistic child compared to women between the ages of 25 and 29.

According to experts, the quality of a woman’s eggs may deteriorate with age, a factor which may lead to difficulty in conceiving as well as abnormalities in birth. This enhances the risk both to the child’s and the mother’s health.

"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," said 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," Shelton added.

Past studies have shown that advanced maternal age is a significant risk factor for a variety of other birth-related conditions such as infertility, early fetal loss, low birth-weight, chromosomal aberrations, and congenital anomalies.

Larger trials needed
Some researchers, however, have countered the findings saying that it is unlikely that all the cases of autism may be linked to a single cause such as parents’ age. Genetic factors, environmental factors could also contribute towards the condition in children, they said.

Therefore, there is a need for larger trials to further an accurate understanding on autism, and its risk factors.

The study appears online in the Journal Autism Research.