Around one in four births in the UK are by Caesarean which, although regarded as very safe, can carry a risk of complications.
These include the risk of infections to new mothers, excessive bleeding and potential damage to surrounding organs.
Risks to the baby include breathing problems, which are fairly common and mostly affect babies born before 39 weeks gestation. This usually improves within a few days.
Of the studies in the new analysis, 23 included women of any weight at the start of the study, seven included obese women only and six were targeted at overweight and obese women.
The researchers, writing in the British Medical Journal (BMJ), found that women of any weight offered tailored diet and exercise advice during pregnancy were less likely to need a Caesarean or gain excessive weight.
There was also some evidence that they were less likely to develop diabetes in pregnancy.
The advice on dieting included restricting sugary drinks, switching to low-fat dairy and eating more fruit and veg.
Exercise programmes included aerobic classes and cycling in the gym, and some weight-based training.
The results showed that dieting combined with physical activity significantly reduced the mother's weight gain during pregnancy by an average of 0.7kg compared with the control group.
The study was led by experts at Queen Mary University of London (QMUL) using data from more than 12,000 women.
One of the authors, Professor Shakila Thangaratinam, said: "Our findings are important because it is often thought that pregnant women shouldn't exercise because it may harm the baby.
"But we show that the babies are not affected by physical activity or dieting, and that there are additional benefits including a reduction in maternal weight gain, diabetes in pregnancy, and the risk of requiring a Caesarean section.
"This should be part of routine advice in pregnancy, given by practitioners as well as midwives."
Prof Thangaratinam said that for every 40 mothers who follow a healthy diet and moderately exercise, there will be one fewer having a Caesarean section.
Changes in lifestyle also reduced the risk of diabetes in pregnancy by 24 per cent.
At present in the UK, only obese women are offered access to a dietitian and specific classes for advice on diet and lifestyle.
But Prof Thangaratinam said: "Often with interventions like these, certain groups benefit more than others, but we've shown that diet and physical activity has a beneficial effect across all groups, irrespective of your body mass index (BMI), age or ethnicity, so these interventions have the potential to benefit a huge number of people."
The results of the study were recently used by the Department of Health, which recommends at least 150 minutes of moderate intensity activity every week.
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