WASHINGTON — Eleven days after her son Benjamin's birth by C-section, Linda Coale awoke in the middle of the night in pain, one leg badly swollen. Just as her doctor returned her phone call asking what to do, she dropped dead from a blood clot.

Pregnancy-related deaths like Coale's appear to have risen nationwide over the past decade, nearly tripling in the state with the most careful count — California. And while they're very rare — about 550 per year out of 4 million births nationally — they're nowhere near as rare as they should be. The maternal mortality rate is four times higher than a goal the federal government set for this year.

"It's unacceptable," said Dr. Mark Chassin of The Joint Commission, the agency that accredits U.S. hospitals and which recently issued an alert to hospitals to take steps to protect mothers-to-be. "Maybe as many as half of these are preventable."

Two years after Coale's death near Annapolis, Md., her sister said topping that list should be warning women about signs of an emergency, like the clot called deep vein thrombosis, or DVT, that can kill if it breaks out of the leg and moves to the lung.

"All she wanted to do was have her own family, and when she finally gets that privilege, she's no longer with us," said Clare Johnson, who insisted her sister's only risk was being pregnant at age 35.

Maternal mortality gets little public attention in the U.S., aside from last year's worry over the swine flu that killed at least 28 pregnant women. Among the leading preventable causes are hemorrhage, DVT-caused pulmonary emboli and uncontrolled blood pressure.

It's not clear what's fueling the overall increase, although better counting is playing some role. But there are some suspects, such as a jump in cesarean deliveries that now account for almost one-third of births. Also, one in five pregnant women is obese, spurring high blood pressure and diabetes, and more women are having babies in their late 30s and beyond.

"It can be a death here, a death there," said Dr. Elliott Main of the California Maternal Quality Care Collaborative, whose research is helping to uncover the rise. "Any one doctor or any one hospital hasn't really seen this change."

When he shows the statistics at medical meetings, "everybody sits up."

More startling, black women are at least three times more likely to die from pregnancy complications than white women, and research is too limited to tell why.

Then there are the near-misses. For every death, 50 additional women suffer serious complications of pregnancy or delivery, said Dr. Jeffrey King of the University of Louisville, a spokesman for the American College of Obstetricians and Gynecologists.

At issue are deaths directly related to pregnancy or childbirth, up to 42 days after delivery. In 2006, the latest year for which data were available, there were 13.3 maternal deaths for every 100,000 births. A decade ago, the rate hovered around 7 — and by this year, the U.S. government had hoped to lower it to 3.3 deaths. California in 2006 charted 16.9 maternal deaths for every 100,000 births, up from a rate of 5.6 in 1996.

How pregnancy-related deaths are coded and counted changed during that time period, but Main said only about 30 percent of the increase might be because of that.