Babies born by caesarean section are more likely to experience obesity than children delivered via vaginal birth, a new study has found.

The study suggests that babies from caesarean deliveries don't only experience greater incidence of obesity as children, but also later on in life – with the effects persisting through teenage years and into adulthood.

"Caesarean deliveries are without a doubt a necessary and lifesaving procedure in many cases," says nutritionist and epidemiologist Jorge Chavarro from Harvard University.

"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."

The researchers analysed data from more than 22,000 young adults who took part in the Growing Up Today Study (GUTS) – an ongoing project launched in 1996 to examine the factors that influence health and weight as people age.

The participants, who were children aged between 9 to 14 when the study commenced, had their body mass index (BMI) tracked over 16 years. The researchers also collected information on factors that might affect obesity, including their mothers' pre-pregnancy BMI, smoking status, and age when the participants were born.

Once these factors were accounted for, the data showed that participants born via caesarean deliveries (22 percent of the group) were 15 percent more likely to be obese on average than those who came into the world via vaginal birth (the other 78 percent).

The association showed up in across genders and different age segments, but was at its strongest when the participants were younger.

Participants born via C-section were 23 percent more likely to be obese when aged 9–12; 16 percent more likely to be obese when aged 13 to 18; and 10 percent more likely when aged 19 to 28.

Strangely, the increased risk of obesity was at its highest when pregnant women had elected to have caesarian births without a documented health reason for the C-section, with their babies found to be 30 percent more likely to experience obesity than children delivered via vaginal birth.

Also of note is the fact that participants in the study with siblings – 12,903 of the overall 22,068 group – who were delivered by caesarian had a 64 percent higher risk of obesity than their brothers or sisters from a vaginal birth.

While researchers have previously observed this association between caesarean sections and obesity, this is the largest and most comprehensive investigation yet into the relationship between birthing methods and future weight outcomes.

"I think that 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 caesarean birth and childhood obesity is real," says Chavarro.

"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."

But the question remains, why is this happening?

At this stage, nobody knows for sure, but children born by caesarian section do have less exposure to their mother's vaginal and gastrointestinal microbiota, and this could play a role in their dietary development and weight as they age.

"Children born via C-section harbour less diverse gut bacteria and these patterns of less diversity have been linked to increased capacity for energy harvest by the gut microbiota," one of the researchers, Audrey Gaskins, told Hannah Devlin at The Guardian. "You can think of it as a slower metabolism."

It's important to remember that, for many women, giving birth via a C-section is a medical necessity to safeguard the health of the mother and her baby – so the researchers are not suggesting that women avoid using this method in a bid to lower their child's risk of developing obesity.

A definitive biological cause is yet to be identified to explain this correlation, so until the link can be confirmed, we need to wait and see what further evidence reveals. But one thing's for sure – getting to the bottom of what's really going on here will be beneficial for all.

"Most often caesarean births are as a result of medical necessity, rather than elective," biomedical researcher Simon Cork from Imperial College London in the UK, who wasn't involved with the study, told The Guardian. "[A]s such, this risk would outweigh any concerns mothers should have regarding the possibility of future weight issues."

The findings are reported in JAMA Pediatrics.