Almost 50% of all cases of gestational diabetes could be prevented if women consumed a healthy diet, maintained a healthy weight, exercised regularly and stopped smoking before and during pregnancy, a new study has found.

Gestational diabetes is a type of diabetes that develops during pregnancy - most often in the second or third trimester. It usually disappears after the baby is born, although women who develop it are at an increased risk of developing diabetes later on. The condition can also lead to risks for the baby, such as excess growth. In such cases, an elective or emergency caesarean may be necessary to deliver the baby.

US researchers set out to assess the effects of a number of modifiable lifestyle factors on the risk of developing gestational diabetes. They looked at over 14,000 healthy women between 1989 and 2001.

Information on the women's diets, weight, exercise levels and smoking status was gathered and cases of gestational diabetes were validated using medical records.

The researchers then used a mathematical formula - known as PAR% - to estimate the proportion of cases of gestational diabetes that hypothetically would not have occurred had all women been in a low-risk group.

During the course of the study, 823 cases of gestational diabetes were diagnosed. The study found that the biggest individual risk factor for this was being overweight or obese prior to pregnancy - in other words, having a body mass index (BMI) higher than 25.

Women who had a BMI higher than 33 were at least four times more likely to develop gestational diabetes compared to women who had a healthy BMI prior to pregnancy.

However, the researchers found that women who maintained a healthy weight, exercised regularly and did not smoke had a 41% reduced risk of developing gestational diabetes compared with other pregnant women.

If women also consumed a healthy diet on top of the other healthy behaviours, their risk of developing gestational diabetes fell by 83%.

Meanwhile, the PAR% of these four risk factors - poor diet, lack of exercise, overweight and smoking - was 47.5%. In other words, almost 48% of all cases of gestational diabetes could have been avoided if women maintained these four healthy lifestyle behaviours.

The researchers from the National Institutes of Health in Maryland acknowledged that it can be difficult to change lifestyle behaviours. However, they suggested that the time before and during pregnancy ‘could represent an opportunity to change diet and lifestyle as these women might be particularly motivated to adhere to advice to improve pregnancy and/or birth outcomes'.

Details of these findings are published in the British Medical Journal.

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