"C-Section babies are no more likely to be obese than those born normally," reports the Mail Online.
The recent rise in the number of babies being born by caesarean section has been linked to the rise in childhood obesity. Previously, scientists have suggested that babies born by caesarean section might have missed out on exposure to beneficial bacteria from a vaginal birth, which might have made them more likely to grow up overweight.
However, this latest study suggests that may not be true.
Researchers followed up more than 97,000 boys born in Sweden between 1982 and 1987, and looked at their weight and height when they were conscripted to military service at 18 years of age. After taking account of their mothers' pre-pregnancy weight, the researchers found that how they were born made no difference to the boys' chances of being obese as they got older.
The study was carried out when both caesarean sections and obesity were less common, which reduces the likelihood of finding a link. Also, because it used conscription to military service as the measuring point, the study only included boys. Conscription was compulsory for boys in the period studied, but not for girls. We do not know if the results of the study would apply to girls.
For practical advice on tackling weight gain in children, visit the NHS website's healthy weight for children section.
The researchers who carried out the study were from the Karolinska Institutet and Umeå University in Sweden. The study was funded by the Stockholm County Council and published in the peer-reviewed journal PLOS Medicine on an open access basis so it is free to read online.
The Times, the Mail Online and The Guardian reported no increased risk of obesity but did not point out that this was based on just 460 boys who had been delivered by caesarean section and were obese at 18 years of age.
This was a cohort study. Cohort studies are good ways to investigate links between factors such as how someone was born and outcomes such as weight in adulthood. They cannot tell us if any particular factor caused a particular outcome. They can tell us if a result is affected by other confounding factors – in this case, the mother's weight.
Researchers used data for all boys born between 1982 and 1987 in Sweden, for whom they could collect information about how they were born (vaginal, planned caesarean or emergency caesarean), and who were conscripted into military service at 18 years of age.
They took their height and weight measurements at conscription to calculate their body mass index (BMI). They then calculated the number of 18 year olds who were obese, overweight or a normal weight, and compared them by birth category.
The researchers took account of potential confounding factors likely to affect the risk of obesity. These included:
They also looked at parental education, and carried out analyses to check for statistical biases. This included looking at a sub-group of boys who had brothers to check for family influences on obesity.
The researchers had information about 97,291 boys. They found that overall, 4.9% of the boys were obese at age 18. Of these, 4.9% (4,334) born by vaginal delivery were obese, 5.5% (229) born by planned caesarean were obese and 5.6% (231) by unplanned caesarean were obese.
However, once the researchers took account of the potential confounding factors, including mothers' pre-pregnancy BMI, there was no difference in the chances of being obese, regardless of how they were born (relative risk ratio (RRR) for planned caesarean versus vaginal birth 1.02, 95% confidence interval (CI) 0.88 to 1.18; RRR for emergency caesarean versus vaginal birth 0.96, 95% CI 0.83 to 1.10). The same was true when looking at the chances of being overweight.
The researchers said: "We observed no association between elective [planned] or nonelective [emergency] CS [caesarean section] and young adult obesity in young male conscripts when accounting for maternal and prenatal factors."
They note that "most of the crude association between CS and obesity could be explained by the maternal pre-pregnancy BMI. This suggests that there is no clinically relevant association between CS and the development of obesity".
There are many reasons why a woman may have a caesarean section. Some previous studies have suggested a caesarean birth might have negative health outcomes for the child. This study should reassure women that having a caesarean section is unlikely to cause a child to become overweight or obese later in life.
While children born by caesarean were slightly more likely to be obese at 18 years of age than those born vaginally, the study suggests that this is not because of how they were born. It may be a reflection of the fact that women who are obese may be more likely to need a caesarean section, and more likely to have children who are overweight or obese. However, the study was not set up to find this out, so we cannot tell for sure.
The main limitation of the study was the low number of boys who had been delivered by caesarean section and were obese at 18 years of age. This reduces the reliability of the findings. The study used information about boys born between 37 and 32 years ago, who would have been 18 between 2000 and 2005. A lot has changed since then and obesity is much more common than it was. The number of caesarean sections has also risen since the 1980s. This raises questions about the relevance of the findings today.
A further limitation was that all the babies in the study were male. We do not know if the results can apply to girls.
Finally, more than half of the boys born during the time period studied were excluded from the study because of missing data about their birth, weight and height, or parents. We do not know how the missing data might have affected the results.