Delivery by caesarean section does not increase the chance of a baby ending up overweight or obese as a young adult, researchers have found, contrary to previous research.

The authors of the study say their work drew on a huge number of people and more fully takes into account a wide range of possible factors that could explain why babies born by caesarean tend to end up heavier.

Once these are considered, they say, the mode of delivery itself is revealed as being unrelated to whether a baby is overweight or obese as a young adult, regardless of whether the caesarean was elective.

“Clinicians and mothers should not be concerned with the mode of delivery as a factor in the development of obesity in the child,” said Viktor Ahlqvist, a co-author of the research from the Karolinska Institute in Sweden.

The number of babies delivered by caesarean is soaring worldwide, and a number of studies have linked the mode of delivery to a range of health conditions in the child, including asthma, allergies and obesity. Possible explanations include the baby missing out on exposure to vaginal microbes.

Such findings have raised concerns that caesarean deliveries may be linked to the epidemic of expanding waistlines. However, the latest research suggests this is not the case. “These two are not linked. The increase in caesarean section globally could not explain the obesity epidemic,” said Ahlqvist.

Dr Audrey Gaskins, of Emory University in the US, whose previous research identified a link to obesity, welcomed the findings. However, she said Sweden and the US might differ in when or how caesareans were performed, and differences in the “obesogenic” environment between the two countries may also explain the conclusions.

Writing in the journal Plos Medicine, Ahlqvist and colleagues report how they drew on data relating to men born between 1982 and 1987 who were conscripted into the Swedish military service before 2006.

On conscription, medical data was collected on the men, including their body mass index (BMI). Data relating to their mode of delivery at birth, as well as characteristics of the mother, was then taken from a national birth register, and data from other records was used to look at parental education levels.

Looking at data for 97,291 men, the team found a 17% greater risk of obesity for those born through non-elective caesarean compared with vaginal delivery. There was no clear association for elective cesarean.

Once the team took into account a host of factors including BMI of the mother before pregnancy, maternal smoking, age of the mother and education level of the parents, the trend was no longer apparent. Indeed, maternal pre-pregnancy BMI was found to be the strongest factor influencing the trend.

In other words, the team say, while men born by caesarean tended to end up slightly heavier as young adults, the mode of delivery was not a factor.

However, the new study has limitations, including that the use of historic data from conscripts meant it involved only men, and subjects did not have severe health conditions.

Dr Pat O’Brien, a spokesperson for the Royal College of Obstetricians and Gynaecologists, said the study suggested factors such as the BMI of the mother before pregnancy explained associations between caesarean birth and obesity in offspring.

“In other words, being overweight is known to increase the risk of both caesarean birth and childhood obesity; this, and other confounding factors, account for the previously suggested link between caesarean birth and offspring obesity,” he said.

O’Brien added that a caesarean delivery could be lifesaving to both mother and baby and that women should be supported however they chose to give birth.

“Women can be reassured that if they have a caesarean birth for any reason, whether planned or emergency, it does not increase the chances that their child will be obese,” he said.