Caesarean-born children appear to perform below their peers in grammar and problem solving, according to a new 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, news.com.au reports.

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.

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"Immediately after caesarean birth, the bacteria present are different from those that are present after a vaginal birth. There's quite a lot of data now indicating that the gut bacteria influence the nervous system," Prof Bornstein said.

"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.

"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", Melbourne University's Cain Polidano told the ABC.

"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.

Dr Charlotte Elder from the Royal Australian and New Zealand College of Obstetricians and Gynaecologists told the Sydney Morning Herald the research was not conclusive enough to trigger changes to birthing practices.

"All we can draw from it is that it is something that needs to be looked at further," she told the SMH.

"With something this complicated, before having a major practice change, you would want multiple studies suggesting the same thing."

Professor Ian Hickie, the co-director at the Brain and Mind Centre at the University of Sydney, told the ABC the results of the study needed to be treated with caution.

"I think there is a real danger here of this going out to the public and saying that we have shown a direct association . . . when really it jumps to some pretty interesting conclusions that this might be due to changes in the gut microbiology, that it might be related to breast feeding or not," Professor Hickie said.

"Lots of other very speculative considerations have been used in the data," he told the ABC.

"At this stage what we know is that the appropriate use of caesarean births saves lives, maternal and child. Also that caesareans are associated with high-risk pregnancies ... because it was a pregnancy at risk in the first place."

Approximately 30 per cent of all births in Australia are by caesarean section - that's higher than the World Health Organisation's recommended figure of 15 per cent for developed countries.

The most recent data in New Zealand shows that 35 per cent of babies born in Auckland City Hospital were delivered via caesarean. The number represented an increase from the 24 per cent of c-section births in 2011.

- Additional reporting by NZ Herald