Los Angeles ? The number of pregnant women with pre-existing diabetes has more than doubled in seven years, a California study found, a troubling trend that means health risks for both mothers and newborns.

And the number of diabetic teenagers giving birth grew fivefold during the same period, according to the study, the largest of its kind.

Expectant mothers who don’t control their diabetes face an increased risk of miscarriage and stillbirth. Their babies have a higher chance of being born with birth defects.

“These are high-risk pregnancies,” said Dr. Florence Brown, an expert on pregnancy and diabetes. “All women with pre-existing diabetes need to plan their pregnancies.”

Brown is co-director of the Joslin-Beth Israel Deaconess Medical Center diabetes and pregnancy program in Boston. She had no role in the study, which was done at Kaiser Permanente, a California-based health care provider.

The researchers focused on health records from more than 175,000 ethnically diverse women who gave birth in a dozen Kaiser hospitals in Southern California from 1999 to 2005. Experts believe the findings likely reflect the overall U.S. population.

The actual number of pregnant women with pre-existing diabetes was small. In 1999, there were 245 such women; by 2005, there were 537. That translates to a rate that rose from 8 per 1,000 pregnancies to 18 per 1,000.

The rate increased the greatest among 13- to 19-year-olds giving birth. It ballooned from about 1 per 1,000 pregnancies to 5.5 per 1,000 during the seven-year period.

Blacks, Asians and Hispanics were more likely to have diabetes before pregnancy than whites.

The rise of diabetes among women of childbearing age mirrors the prevalence of the disease in the general population. The most common form of diabetes is Type 2, which is linked to obesity.