Babies born by caesarean section are 15 per cent more likely to become obese as children compared to those born naturally, a study by Harvard University has found.
The study also showed that children born by c-section had a 64 per cent greater chance of becoming obese when compared with natural born siblings, despite growing up in the same environment.
"Caesarean deliveries are without a doubt a necessary and lifesaving procedure in many cases," said Dr Jorge Chavarro, associate Professor of Nutrition and Epidemiology at Harvard Chan School of Medicine, the senior author of the study.
"But cesareans also have some known risks to the mother and the newborn. Our findings show that risk of obesity in the offspring could another factor to consider.
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"Our findings particularly those that show a dramatic difference in obesity risk between those born via caesarean and their siblings born through vaginal delivery provide very compelling evidence that the association between cesarean birth and childhood obesity is real.
"That's because, in the case of siblings, many of the factors that could potentially be playing a role in obesity risk, including genetics, would be largely the same for each sibling - except for the type of delivery."
It is the first study to show a clear link between obesity and c-sections and involved more than 22,000 people over 16 years.
The researchers say the findings may be related to differences in the gut bacteria which are set at birth. Babies born vaginally have greater exposure to their mother's vaginal and gastrointestinal bacteria which are known to be beneficial.
A recent study showed that babies born by caesarean lack an important group of bacteria called bacteroidetes which suppress the immune system. Without the microbes, the immune system can over-react leading to allergies, diabetes and inflammatory bowel disease.
The research was published in the journal JAMA Paediatrics.