In the United States the rate of cesarean delivery (CD) has risen 48% since 1996, reaching a level of 31.8% in 2007. This trend is reflected in many parts of the world, with the most populous country in the world, China, approaching 50% and some private clinics in Brazil are approaching 80%. While a significant number of CD are preformed for obstetrical indications, most are simply due to maternal request and may incur several risks for the child.
Most women are unaware that babies born by elective C-section are much more likely to develop health problems that many newborns who are delivered naturally do not experience. The babies may miss out on critical hormonal and physiological changes during labour which help babies develop.
A Danish study examining 34,000 deliveries suggests babies born by C-section were up to four times more likely to have respiratory problems than those born naturally.
A recent study also showed that caesarean born babies are also at double the risk of becoming obese children as those delivered naturally.
Women who have their first child by caesarean are also more likely to have placenta-related problems in their second pregnancy, research suggests.
Until fairly recently, babies were thought to be born with sterile guts free from bacteria. But we now know that babies are born with a gutful of microbes, and that at least some of these come from a mother's vaginal canal during birth.
Babies born by C-section are thought to miss out on these bacteria, which could explain why their microbiomes look different. The ecosystem of microbes that live inside us has been implicated in a range of health issues, so this may explain why babies born by C-section are more likely to grow up overweight, and to develop allergies and asthma in later life.
To test if C-sections really do lead to heavier babies, Maria Dominguez-Bello at New York University and her colleagues performed C-sections on 34 pregnant mice, and compared the resulting pups to 35 that were born vaginally. By the time the mice had grown into adults 15 weeks later, there were stark difference in body weight between the two groups. The mice born by C-section were, on average, 33 per cent heavier than those born vaginally.
Females seemed particularly affected, says Dominguez-Bello. While the C-section males were around 20 per cent heavier than their vaginally-born counterparts, the females were 70 per cent heavier, she says. "We were very surprised to see this," she says. "We have no idea why it's happening."
The microbiomes of the C-section mice also looked different to those born vaginally. By the time they were four weeks old, the C-section mice had lower levels of some bacterial species and more of others. The team don't know whether these changes might impact the mice's health.
While the C-section mice are heavier, it isn't clear whether they are actually unhealthy. They don't have a higher proportion of body fat -- rather, it seems that their bones, fat and tissues just grow much more than vaginally-born mice.
The team think that being deprived of the bacteria in the vaginal canal could somehow prime pups -- and possibly human babies -- for obesity later in life.
Phillip Bennett of Imperial College London isn't convinced the findings apply to humans. Mice have a diverse range of vaginal bacteria, while in people, the vaginal microbiome is largely dominated by a single strain.
At any rate, C-section births don't hugely increase the risk of obesity in human children, says Bennett. "You're 1.3 times more likely to be obese if you're delivered by Caesarean section than vaginally," he says. But this statistic might be warped by the fact that obese mothers are more likely to require the surgery, as are women who are pregnant with larger babies.
Dominguez-Bello and her colleagues have been swabbing newborn C-section babies with vaginal fluids from their mothers to see if this has a beneficial effect. She doesn't recommend the practice though, because she doesn't know yet if it works, and there is a risk that the procedure could cause infections.