Experts recommend pregnant women do at least 150 minutes of moderate intensity exercise each week to help reduce the risk of a C-section and diabetes.

Pregnant women who have a healthy diet and participate in regular moderate exercise are less likely to have a caesarean section.

They are also less likely to gain excessive weight or develop gestational diabetes, according to a longitudinal study of more than 12,000 women in the UK.

Experts say the findings published in the journal BMJ show that exercise during pregnancy is safe, and recommend that women perform at least 150 minutes of moderate intensity activity every week.

"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," said lead researcher Professor Shakila Thangaratinam from Queen Mary University of London.

"This should be part of routine advice in pregnancy, given by practitioners as well as midwives. Now that we're able to link the advice to why it's beneficial for mothers-to-be, we hope mothers are more likely to adopt these lifestyle changes," Professor Thangaratinam said.

Researchers from the International Weight Management in Pregnancy (i-WIP) Collaborative Group assessed diet and physical activity based interventions in pregnancy, and how they impacted on weight gain and outcomes such as caesarean section, stillbirth and admission to a newborn intensive care unit.

Interventions included access to a dietician, specific antenatal classes for advice on diet and lifestyle, or structured exercise of moderate intensity, such as aerobic classes or stationary cycling.

The research team analysed individual participant data from 36 randomised trials involving about 12,500 women.

Diet and physical activity based interventions consistently reduced gestational weight gain by an average of 0.7 kg, regardless of age, pregnancy history, ethnicity, BMI, and existing medical conditions.

Interventions also lowered the odds of caesarean section. According to the findings, the odds of the mother having a caesarean section reduced by about 10 per cent.

"For every 40 mothers who follow the healthy diet and moderate exercise, one less woman will end up with a caesarean section," Professor Thangaratinam said.

Changes in lifestyle reduced the risk of diabetes in pregnancy by 24 per cent, a condition which normally affects over 1 in 10 mothers in pregnancy and increases risks of complications in mother and baby.

The interventions also had no effect on offspring outcomes, such as stillbirth or admission to an intensive care unit.