Babies who were born by caesarean section are 33 per cent more likely to be diagnosed with autism spectrum disorder, according to a new analysis of more than 20 million births.

The controversial study, which included Australian data, also found a 17 per cent higher chance of developing attention deficit hyperactivity disorder (ADHD) among children delivered by C-section.

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Whether the surgery was elective or emergency made almost no difference to the odds of developing these disorders, says the study published by JAMA Network Open.

"The results appear to further add to the known adverse health outcomes associated with cesarean delivery and suggest judicious use of cesarean delivery," it says.

"Future studies on the mechanisms behind these associations appear to be warranted."

The study reviewed more than 60 individual studies looking at more than 20 million babies born in 19 countries since the 1960s.

Study 'flawed'

Swinburne University associate professor Jason Howitt says the study is "significantly flawed" because some of the studies showed autism rates well above the worldwide average.

"These findings should not alter any parent’s decision on the type of delivery method for their child," Howitt said.

"Nor should any mother who has an autistic child think that their birth choice could have caused autism, this is simply not true."

Other experts warn the data does not consider factors like the mother's age, the child's sex, gestation time or breastfeeding practices.

Chevron Right Icon 'Garbage in, garbage out.'

University of Adelaide emeritus professor Alastair MacLennan says combining so many different studies makes the study's conclusion "very unreliable".

"The old saying from computer science: 'garbage in, garbage out' seems to apply," MacLennan said.

"Genetic causes are now being confirmed for many of these ... disorders."

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University of Western Australia professor Jeffrey Keelan says the study is "interesting and well-performed".

"The big question is: what is the causal connection, if any, underpinning these findings?" he said.

"As openly acknowledged by the authors, further research is needed."

Chevron Right Icon There is no consensus.

Melbourne IVF clinical director Alex Polyakov also questioned the wisdom of analysing so many different studies together.

"Both caesarean section and vaginal delivery have risks for mothers and babies," Polyakov said.

"These risks are not the same and there is no consensus as to which mode of delivery is better overall."

The study, which included data from more than 300,000 Australian cases, was compiled by Tianyang Zhang of Sweden's Karolinska Institute.