Babies born by Caesarean section are twice as likely to be obese in childhood, researchers claim.
Nearly 16% of children delivered via Caesarean section were obese by the age of three compared with 7.5% of those born by a normal vaginal birth, according to research published online in the Archives of Disease in Childhood.
Although the study does not look for explanations behind the link, researchers speculate that it could be caused by the composition of gut bacteria acquired at birth.
Previous research has identified different levels of bacteria in the guts of children born vaginally and by Caesarean.
Studies have found that children born by Caesarean delivery have higher numbers of Firmicutes bacteria. Other research found that obese people have higher levels of the same bacteria in their guts.
US researchers studied 1,255 mother and child pairs in eastern Massachusetts between 1999 and 2002.
One in four of the children was born by Caesarean section. They were all measured and weighed at birth, six months and three years.
"An association between Caesarean birth and increased risk of childhood obesity would provide an important rationale to avoid non-medically indicated Caesarean section," write the authors.
"Expectant mothers choosing Caesarean delivery in the absence of an obstetrical or medical indication should be aware that their children may have a higher risk of obesity."
Sue Macdonald, education and research manager at the Royal College of Midwives, warned that unnecessary Caesarean sections have detrimental effects.
She said: "This highlights the need to avoid Caesarean sections that are not medically needed.
"Evidence is building and suggests that unnecessary Caesarean sections may have detrimental effects in the short and long term for the woman and the child. This research adds to that evidence but further research is needed to confirm these findings.
"There is also a need to provide on-going support to women, their babies and families towards improving and addressing weight management, initially through supporting breastfeeding and then through healthy eating.
"This can have benefits not just around pregnancy but for the mother and her family generally."