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

Dr. 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 by researchers at a California-based health care provider, Kaiser Permanente.

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 between 1999 and 2005. Experts believe the findings likely reflect the overall American 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.

About 15 million people in America are diagnosed with diabetes, and 1.5 million new cases were diagnosed in people age 20 and older in 2005, according to the American Diabetes Association.

Pre-pregnancy diabetes is different from gestational diabetes, which is developed during pregnancy and disappears later. Gestational diabetes affects 3% to 8% of pregnant women in America.

In the study, Kaiser researchers did not look at whether any of the women had prenatal diabetes care or how the babies fared after birth. They also could not determine the type of diabetes the women had.

Type 2, which is linked to obesity, occurs when the body makes too little insulin or cannot use what it does produce. Type 1 occurs when the body doesn’t produce insulin.

Results of the study were published online today in the journal Diabetes Care, a publication of American Diabetes Association, which funded the research.