"The baby travelling through the birth canal is going to get the right sorts of bacteria" ... study co-author Charles Algert.
Babies born by caesarean are much more likely to be admitted to hospital with gastrointestinal disease or chest infections in their first year of life than those born naturally, a study of NSW births has found.
The babies were 22 per cent to 26 per cent more likely to be hospitalised with gastrointestinal disease and about 12 per cent more likely to be admitted with bronchiolitis, a type of chest infection, the researchers, from the Kolling institute at Royal North Shore Hospital, found.
The study co-author Charles Algert said children born by caesarean could miss out on picking up important gut bacteria that children born naturally get during the birth.
"We take all these yoghurts and things to get the right bacteria in our guts but the baby travelling through the birth canal is going to get the right sorts of bacteria," he said.
However, there could also be a link between caesareans and breastfeeding problems.
The study, which analysed data from 626,700 births in NSW between 2001 and 2008, found women who gave birth by caesarean were 70 per cent more likely to be diagnosed with a complication affecting breastfeeding.
And the babies of the women with breastfeeding problems were then 30 per cent more likely to be hospitalised with gastrointestinal problems.
Caesarean rates are increasing in NSW. In this study, published in the journal Archives of Diseases in Childhood, about 26 per cent of the babies were born by caesarean.
According to the most recent NSW Health Mothers and Babies report in 2009 the caesarean rate was more than 30 per cent, and as high as 45 per cent in some private hospitals.
Mr Algert said there was debate about the mechanisms through which caesareans were linked to poorer health.
Earlier Australian research had found the link between bronchiolitis and caesareans existed with only planned caesareans, suggesting labour itself could activate the mothers' immune system.
But Mr Algert said his research indicated it was the birth that was important.