List of risk factors from the Mayo Clinic.
Age greater than 25. Women older than age 25 are more likely to develop gestational diabetes.
Family or personal health history. Your risk of developing gestational diabetes increases if you have prediabetes — slightly elevated blood sugar that may be a precursor to type 2 diabetes — or if a close family member, such as a parent or sibling, has type 2 diabetes. You're also more likely to develop gestational diabetes if you had it during a previous pregnancy, if you delivered a baby who weighed more than 9 pounds (4.1 kilograms), or if you had an unexplained stillbirth.
Excess weight. You're more likely to develop gestational diabetes if you're significantly overweight with a body mass index (BMI) of 30 or higher.
Nonwhite race. For reasons that aren't clear, women who are black, Hispanic, American Indian or Asian are more likely to develop gestational diabetes.
What I'd like to know though, is how much each factor increases your risk, and what your overall risk would be with none of the factors.

