Babies born via elective caesarean are more likely to be overweight at 12 months of age, new research shows.
Caesarean rates have more than doubled globally in the past 20 years.
The study, led by researchers from the Duke-NUS Medical School in Singapore, draws on data from an ongoing longtitudinal study in that country.
Data from 727 babies and their mothers was reviewed, and of the 222 babies born via caesarean, 74 were from elective procedures. It found elective caesarean was significantly associated with being overweight or at risk of overweight at the age of one.
The international research team included child obesity expert Professor Sir Peter Gluckman from the University of Auckland-based Liggins Institute.
Sir Peter told Nine to Noon there did seem to be a causal link with elective caesarean births, as the control groups - those who had a normal births or emergency caesareans - did not have the unusually high levels of obesity.
He said hormonal changes in the the 36 hours before birth, and particularly the last few hours, made important changes to the baby's development, and may set up metabolic changes that can lead to obesity.
Sir Peter said it could also be that the baby delivered by elective caesarean was not exposed to bacteria and other organisms.
"It may also be that the bacteria, the microbiological organisms in the vagina during labour, are already starting to creep up through the opening cervix to invade the baby.
"And that the failure to be exposed to the vaginal organisms during either labour or vaginal delivery is changing the microbiome of the baby in such a way that they end up obese."
Sir Peter said they could not separate the two factors with this study, and he thinks both might be involved.
Work was being done in Singapore and at the Liggins Institute to try to look at those matters.
Sir Peter said the risks to babies and mothers from caesareans are well known, but what was new from this study is the indication that it was not just the acute health of the baby in the few days after birth that was affected by the mode of delivery, but long term consequences are also starting to emerge.
"I'm not going to claim that the reason to not do caesareans section is due to an increase of body fat at one year of age, there are many of other reason to think about the choice of delivery.
"I think this is a red flag to say we need to think more about the consequences of avoiding the evolved way of giving birth, namely through the vagina."