A causal link involving friendly bacteria has been found between C-section births and increased obesity risk.
Scientists had already shown that Caesarean babies are more likely to grow up obese, but not been clear about the reason why.
The new research, conducted on mice, confirms that the weight gain is linked to the impact of C-section birth on gut bugs.
Lead scientist Dr Maria Dominguez-Bello, from New York University School of Medicine, said: "Our study is the first to demonstrate a causal relationship between C-section and increased body weight in mammals."
The team compared 34 mice delivered by C-section with 35 animals born naturally while following their weight changes and analysing intestinal bacteria.
The make-up of the "microbiome" - bacterial population - in vaginally born mice evolved normally. C-section mice not only gained weight but also had colonies of gut bugs that did not follow the usual progression path.
The study, published in the journal Science Advances, also showed that normal "pioneer" bacteria transferred from mothers to offspring via the reproductive tract appeared to protect against weight gain.
Dr Dominguez-Bello added: "The question of whether a baby's founding microbiome affects its future obesity risk becomes more urgent as C-sections are increasingly used by choice in many parts of the world.
"Further research is needed to determine whether the dominance of certain bacterial groups can protect against obesity."
