Caesarean-born children appear to perform below their peers in grammar and problem solving, according to a world-first study.
Researchers from the University of Melbourne measured the cognitive performance of 5000 Australian children who were born vaginally and via caesarean using NAPLAN results and data from the Longitudinal Study of Australian Children.
The study, which was published in the peer-reviewed journal Scientific Reports, revealed different bacteria in the guts of babies delivered by caesarean could be behind the delay.
"Caesarean birth is associated with a different colonisation of the gut by the bacteria that form a large ecosystem inside the gastrointestinal tract," Professor of physiology Joel Bornstein told the ABC.
"Immediately after caesarean birth, the bacteria present are different from those that are present after a vaginal birth," Prof Bornstein said.
"There's quite a lot of data now indicating that the gut bacteria influence the nervous system.
"So we believe, although there's no way of proving it at this point, that this may be the difference that leads to the cognitive changes later on in life."
Children born via C-section had small delays in their grammar, numeracy, reading and writing skills between the ages of four to nine.
Melbourne University's Cain Polidano told the ABC: "There is already a bit of evidence that shows that caesarean birth is related to a number of negative childhood health outcomes, including risks of ADHD, autism and also asthma.
"So our research speaks to that literature which shows that there's a link, but what we do now is look at impacts on another outcome, which is child development," Dr Polidano said.
The researchers said their study underlined the need for a precautionary approach in responding to requests for a planned caesarean when there are no apparent elevated risks from vaginal birth.
About 30% of all births in Australia are by caesarean section, much higher than the World Health Organisation's recommended figure of 15% for developed countries.
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