A US study found that babies born via caesarean section had a 64% increased risk of becoming obese compared to their siblings born by vaginal delivery.
The study used responses from more than 22,000 children and young adults, and their mothers (all of whom were nurses) over a 16 year period.
Ultimately, the study doesn't prove that caesarean section causes obesity. An important limitation of the study, acknowledged by the researchers, is that they did not know the reasons why a caesarean section was carried out in the first place.
While the researchers tried to adjust their results for known risk factors for obesity in children, such as maternal age and pre-pregnancy weight, it is likely that other, unaccounted for, factors contributed to the risk of obesity. For example, maternal diet and breastfeeding were not investigated, which are known to influence children's weight.
The researchers do recommend that the potential increased risk of obesity should be explained to women considering having a caesarean section by choice (as opposed to the procedure being required due to a complicated labour).
Any increase in potential risk is just that; a possibility, not a guarantee set in stone. You can offset the increase by ensuring your child eats a healthy diet and is physically active. Read more advice about helping your child maintain a healthy weight and what you can do if you are concerned they may be overweight.
The study was carried out by researchers from several US institutions including the Harvard TH Chan School of Public Health, the Brigham and Women's Hospital and Harvard Medical School and The Dartmouth Institute for Health Policy and Clinical Practice. Funding came from the National Institutes of Health, US. There were no conflicts of interest declared by the authors.
The study was published in the peer-reviewed journal JAMA Pediatrics.
While the UK media's reporting of the story was generally accurate, The Sun included an unhelpful section listing potential risks of having a caesarean without clarifying that the risk of some of these outcomes (such as bladder damage) is extremely small.
This was a prospective cohort study using data collected from questionnaires completed by people in the US. These studies are a good way to find links between factors, in this case caesarean birth and obesity in childhood, adolescence and early adulthood. They cannot prove that one causes the other - that caesarean section caused obesity, however, as an experimental study would not be ethical, this is the best study design for finding a possible link.
Researchers analysed data on 22,068 young adults from an ongoing study in the US called the Growing Up Today Study. Participants started completing questionnaires aged 9 to 14 in either 1996 or 2004 and continued to complete a questionnaire every one or two years until 2012.
Each follow up questionnaire asked participants' height and weight. Participants' mothers undertook a questionnaire in 2009 that asked them to recall factors such as mode of delivery (vaginal or caesarean), BMI before pregnancy, diabetes during pregnancy, smoking and the age they gave birth.
In the data analysis, the authors adjusted the results to take into account many of these factors which could have influenced the mode of delivery and future obesity in the child.
Of the 22,068 children questioned, 4,921 had been born by caesarean section. Among all participants, there was a 13% risk of obesity by the end of follow up, aged 20 to 28.
The researchers concluded there was an "association between caesarean delivery and increased risk of obesity in offspring that persisted through early adult life."
They also report "for the first time, to our knowledge, a protective effect of vaginal birth after caesarean delivery on obesity in offspring and a significant difference in risk of obesity between siblings whose modes of birth were discordant."
The researchers suggest that the findings may be related to differences in gut bacteria which are set at birth. Babies born by vaginal delivery have exposure to different bacteria which are known to be beneficial.
The authors have shown that there appears to be a link between mode of childbirth and obesity later in life for offspring.
The strengths of the study were that it was a large prospective cohort that examined BMI over a long period, meaning the risk of obesity could be seen from childhood into early adulthood. The reporting of information on pregnancy also allowed other factors to be accounted for.
However, there are a number of important considerations:
While it may be the case that some people born via caesarean have an increased tendency towards obesity, such a tendency can be overcome through the standard pattern of healthy eating and regular exercise.
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