Researchers say the strongest individual risk factor in women who develop gestational diabetes is being overweight or obese before pregnancy.

Nearly half of all cases of diabetes during pregnancy can be prevented if young women eat well, exercise regularly and stop smoking before and during pregnancy, a study shows.

Gestational diabetes is a common pregnancy complication that has long-term adverse health implications for both mothers and babies.

A team of researchers based in the US set out to examine the effect of lifestyle factors on the risk of developing the condition.

They found the strongest individual risk factor was being overweight or obese - having a body mass index (BMI) above 25 - before pregnancy.

Women with a BMI above 33 were more than four times more likely to develop gestational diabetes than women who had a normal BMI before pregnancy.

Women who had a combination of three low risk factors (not smoking, engaging in regular physical activity, and being a healthy weight) were 41 per cent less likely to develop the condition than other pregnant women.

This figure rose to 52 per cent if they began their pregnancy at normal weight.

Importantly though, not smoking, eating well and exercising were associated with substantial benefit even for women who were overweight or obese before pregnancy.

The percentage of the four risk factors in combination (smoking, inactivity, being overweight and poor diet) was 47.5 per cent - indicating an estimated 48 per cent of all gestational diabetes pregnancies could have been avoided if women adhered to all four pre-pregnancy lifestyle factors.

The study, published on thebmj.com, provides researchers with valuable information, says associate professor Sara Meltzer at McGill University, Montreal, Canada.

She asks whether doctors should encourage all women planning pregnancy to adopt these healthier lifestyles, or should attempts be limited to those at higher risk.

Although successful modification of diet, exercise, body weight and smoking habits are not easy for anyone, these findings "should give health professionals and women planning a pregnancy the encouragement they need to try even harder", she concludes.

Copyright AAP 2014.