Professor Neena Modi, from Imperial College London, said: "There are good reasons why C-section may be the best option for many mothers and their babies, and C-sections can on occasion be life-saving. However, we need to understand the long-term outcomes in order to provide the best advice to women who are considering Caesarean delivery.
"This study shows that babies born by C-section are more likely to be overweight or obese later in life. We now need to determine whether this is the result of the C-section, or if other reasons explain the association."
Caesareans now account for up to a third of births in England, twice as many as in 1990.
In some countries the C-section rate is much higher, with 60 pre cent of mothers in China and almost half in Brazil undergoing the procedure.
Previous research has linked other adverse outcomes in childhood including asthma and insulin-dependent Type 1 diabetes to Caesarean birth.
The new study pooled together results from 15 separate studies conducted in 10 countries.
This kind of "meta-analysis" can often uncover patterns that only emerge from large amounts of data.
The research showed that adults born by C-section have a BMI around half a unit higher on average than those born by vaginal delivery.
BMI is a widely used measurement that relates weight and height. It defines the parameters for being overweight or clinically obese.
The study authors wrote in the online journal Public Library of Science ONE: "There is a strong association between CS (Caesarean section) and increased offspring BMI, overweight and obesity in adulthood."
They acknowledge that unknown factors unrelated to childbirth might help explain the findings, but point out that the trend appears to support previous work.
Dr Matthew Hyde, one of the researchers, said: "There are plausible mechanisms by which Caesarean delivery might influence later body weight. The types of healthy bacteria in the gut differ in babies born by Caesarean and vaginal delivery, which can have broad effects on health.
"Also, the compression of the baby during vaginal birth appears to influence which genes are switched on, and this could have a long-term effect on metabolism."
Mervi Jokinen, from the Royal College of Midwives (RCM), said: "There is increasing evidence about the negative implications for women and their babies of having a Caesarean section. This research provides more information for health professionals to review and discuss with women. A decision to have a Caesarean section should not be taken lightly by women or doctors.
"Whilst some Caesarean sections are needed for medical reasons, many are not. We would encourage women to think carefully and weigh up the evidence before they decide to have a non-urgent Caesarean.
"Women should also be aware that this is a major surgical operation that has the potential for increased complications every time a woman has the procedure carried out."
She said the RCM supported guidelines from the NHS watchdog the National Institute for Health and Clinical Excellence (Nice) which encourage women to have a "thorough discussion" with health professionals about the implications of C-section delivery.
- PAA
