Smoking during pregnancy is a major risk factor for attention deficit hyperactivity disorder in the child, WA research shows.
In one of the biggest population studies of ADHD, the Telethon Institute for Child Health Research tracked almost 13,000 children with the learning disorder and found their mothers were much more likely to be younger, single, have smoked during pregnancy and had complications in pregnancy and birth.
The results, published in the international journal Pediatrics, showed no difference if the child was a girl or boy.
ADHD is the most-common neurodevelopment disorder, affecting more than one in 20 children worldwide and leading to learning and social problems that can last a lifetime.
Researchers said though there was a strong genetic predisposition to the condition, early environmental and maternal factors also played a part.
Lead researcher Desiree Silva said the study used a careful clinical definition of ADHD and linked a range of data that overcame many limitations of past studies.
"We found that certain maternal factors did increase the risk of ADHD in the child, with smoking during pregnancy being a significant risk factor," she said.
"Other factors such as low birth weight, giving birth at greater than full term, and low Apgar scores in the baby were not associated with an increased risk of ADHD in the child."
Dr Silva said smoking in pregnancy was an important risk factor for many health outcomes for the mother and the baby, and women should be encouraged not to smoke during pregnancy.
"Attention to maternal infections and pre-eclampsia may also reduce this risk of ADHD," she said.
But Dr Silva said the study had identified broad risk factors rather than causes, and the information could not be used to identify factors associated with any particular child's disorder. More studies were needed, she said.