A new study indicates that diabetes during pregnancy may increase the risk of heart disease.

The study suggests that women with a history of diabetes during pregnancy (gestational diabetes) are twice as likely by mid-life to develop calcium in heart arteries -- a strong predictor of heart disease -- even if healthy blood sugar levels were attained many years after pregnancy. "We were surprised to discover that women with a history of gestational diabetes are at a significantly greater risk of heart artery calcification, even if they maintain normal blood sugar levels after pregnancy," said co-author Erica P. Gunderson from Kaiser Permanente in California, US.

For the study, published in Circulation: Journal of the American Heart Association, the team enrolled approximately 1,100 women without Type 1 or Type 2 diabetes who subsequently gave birth at least once during the 25-year study period, which ended in 2011. Blood tests were performed from before to after pregnancies at five-year intervals to determine if women had normal blood sugar levels, intermediate elevations in blood sugar levels (pre-diabetes) or they had developed overt Type 2 diabetes.

Heart scans were performed to measure coronary artery calcium, a strong predictor for heart disease, at exams 15, 20 and 25 years after the baseline, the first exam of the study. At the 25-year follow-up, the participants' median age was 48 years, and 12 percent of the women in the study had a pregnancy complicated by gestational diabetes. The prospective analysis found that women with a history of gestational diabetes had a two-fold higher risk of coronary artery calcification whether they had healthy blood sugar levels, pre-diabetes or Type 2 diabetes. Of women with previous gestational diabetes, 36 percent developed pre-diabetes and 26 percent developed Type 2 diabetes, compared to 35 percent and 9 percent of women with no history of gestational diabetes, the researchers said.