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 a 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,” says 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 says 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.

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: A jump in cesarean deliveries that now account for almost a third of births. One in five pregnant women is obese, spurring high blood pressure and diabetes. More women are having babies in their late 30s and beyond.

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, notes Dr. Jeffrey King, a spokesman for the American College of Obstetricians and Gynecologists.