Exercise programmes included aerobic classes, cycling in the gym, and some weight-based training.

Exercising during pregnancy is safe for babies and can cut a woman’s chance of needing a Caesarean section by a tenth, say researchers.

Mothers-to-be who do gentle exercise such as aerobics, walking or using a bike machine, and eat a healthy diet are more likely to deliver their baby naturally, suggests evidence from 12 500 women in 16 countries.

Around one in four births are by Caesarean which, although regarded as very safe, can carry a risk of complications. These include infections in new mothers, excessive bleeding and potential damage to surrounding organs. There are also risks to the baby, include breathing problems, although these usually improve within a few days.

Previous research has found 47 percent of women put on too much weight over the nine months of their pregnancy, raising the chance of a needing a C-section.

Experts think this is because too many women wrongly believe they should "eat for two" during pregnancy.

The latest research, by experts at Queen Mary University of London, assessed data from 36 previous clinical trials. 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, 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. They were also 24 percent 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, 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 1.5lb (0.7kg) compared with other women.

Professor Shakila Thangaratinam of Queen Mary University said: "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." Professor Thangaratinam said that for every 40 mothers who follow a healthy diet and moderately exercise, there will be one fewer having a Caesarean section.

The Royal College of Obstetricians and Gynaecologists advises that pregnant women do up to 30 minutes a day of aerobic exercise – such as running, swimming or dancing.

But the Royal College say women should not expect to be as fit as they were before the pregnancy, and should stop exercising as soon as they feel unwell.

It advises: "During recreational exercise, you should be able to hold a conversation. If you become breathless as you talk, then you are probably exercising too strenuously."