Previous studies have linked smoking to the risk of type 2 diabetes, the study's authors point out in the American Journal of Epidemiology, but the relationship between smoking and gestational diabetes remains unclear.

Dr. Lucinda J. England from National Institute of Child Health and Human Development in Bethesda, Maryland, and colleagues used data from 4,500 pregnant women to investigate.

Average blood glucose levels were highest among women who currently smoked, the team reports, and were lowest among women who never smoked or who quit before or during pregnancy.

Rates of gestational diabetes were highest among women who smoked (4.4 percent) and lowest among those who had never smoked (1.8 percent), the investigators found. Women who quit before or during pregnancy had intermediate rates of gestational diabetes (1.9 percent and 2.5 percent).

If in fact smoking causes gestational diabetes, the researchers write, "then 47 percent of gestational diabetes mellitus in smokers and 10 percent in all women in our study population could potentially be attributed to tobacco exposure."

Thus, they conclude, "Smoking may be an important modifiable risk factor for gestational diabetes mellitus."