Washington, Feb 9 : A new study has revealed that women who become mothers at an advanced age are 50percent more likely to have autistic children.

The study led by UC Davis Health System researchers showed that 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.

It showed 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.

Advanced paternal age is also known as a risk factor for having a child with autism, however, the elevated risk is only when the father is older and the mother is under 30.

For example, one study reported that fathers over 40 were six times more likely than fathers under 30 to have a child with 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," 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.

Earlier studies have observed that advanced maternal age is a risk factor for a variety of other birth-related conditions, including infertility, early fetal loss, low birth-weight, chromosomal aberrations and congenital anomalies.

The study is published online in the journal Autism Research.