Exercise before and during pregnancy can greatly reduce the risk of gestational diabetes.

Researchers at the Harvard School of Public Health, Boston, US, showed physical activity before pregnancy can lower gestational diabetes risk by up to half.

They also found exercise in early pregnancy cuts the risk by a quarter. Researchers examined studies of 34,929 patients' activity levels prior to pregnancy and rates of gestational diabetes.

Women in the highest physical activity quartiles prior to pregnancy were 55 per cent less likely to develop gestational diabetes than those in the lowest. This benefit dropped to 24 per cent in those exercising in early pregnancy.

David Stalker, executive director, Fitness Industry Association (FIA), said: "Pregnant women must not only be supported to exercise during the post natal stage, when they are trying to get back to their pre pregnancy figure, but also gentle exercise during pregnancy too. The research points to the benefits.

"Engaging more women in more physical activity and educating them around the health benefits of participation is essential for lowering the risk of conditions such as this. There are lots of different classes to meet individual needs - from pregnancy yoga to swimming to moderate walking - it's just a case of working out what suits individual needs at each stage of the pregnancy".

Deepa Khatri, clinical adviser at Diabetes UK, said: "Women should try to achieve a healthy weight before conception, as active weight reduction during pregnancy is not appropriate. But it is important to encourage expectant mothers to avoid excess weight gain."

NICE recommends pregnant women undertake 30 minutes of moderate intensity exercise per day.