Mums in 40s face 50pc higher autism risk
DELAYING motherhood increases a woman's chances of having an autistic child, a study has shown.
At the age of 40 a woman is 50pc more likely to give birth to a child with the developmental disorder than she is in her mid- to-late-20s, say scientists.
Previously, the risk of autism was primarily linked to the age of the father, but new research from the US shows a father's age is only important when he has a young wife.
In contrast, being an older mother significantly increases the risk of having an autistic child regardless of the father's age.
Researcher Janie Shelton, from the University of California at Davis (UC Davis), said: "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.
"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. Among mothers over 30, increases in the father's age do not appear to further increase the risk of autism."
Abnormal brain development in the womb is thought to trigger autism, which mostly affects boys.
Around one in 100 people in the UK are believed to be affected by Autism Spectrum Disorder, which covers a range of related conditions. An estimated 133,500 people under the age of 18 in the UK suffer from autism. The US scientists based their findings on birth and health records for almost five million individuals born in the 1990s. In total, they identified 12,159 cases of autism.
During that decade, the number of Californian women over 40 becoming mothers rose by more than 300pc.
Over the same period, there was a 600pc increase in the number of children in the state diagnosed with autism. The researchers estimated that women waiting longer to have children accounted for around 5pc of this trend. For older mothers, a step-wise progression in the risk of giving birth to an autistic child was apparent whatever the age of the father.
But having an older father increased the risk only when the mother was young. Among births to mothers under 25, children fathered by men of more than 40 were twice as likely to develop autism as those whose fathers were aged 25 to 29.
Professor Irva Hertz-Picciotto, from the UC Davis MIND Institute, said: "We still need to figure out what it is about older parents that puts their children at greater risk of autism and other adverse outcomes, so that we can begin to design interventions."
A possible clue may have emerged from a 2008 study suggesting that an immune response from the mother may be involved, said the scientists.
Some mothers of autistic children were found to produce antibodies against foetal brain protein, while this was not seen in any mothers of unaffected children. Advancing age has separately been associated with an increase in auto-immune reactions.