According to extensive research conducted in the United States, a mature age of the mother increases the risk of autism in a child, regardless of age of his father.

Scientists have shown that increasing maternal age by 5 years is associated with increased risk of autism in children by 18%. Thus, for a woman who became a mother at the age of 40 years, the risk of birth of child with autism is 100% higher than in women in 25 years of age.
These results disprove the previous hypothesis that a key risk factor for autism in a child is the father’s age. The study showed that women at the age of 35 and higher age are always have a high risk. A middle age of fathers is a risk factor in case when mother is much younger than his father (under 30 years).
The study recorded basic information about all the babies and their parents in California beginning from the 1^st of January 1990 and December 31, 1999. Results were examined 4.9 million births and 12 159 cases of autism.

The study is published in the online version of the journal Autism Research.