More than 540,000 babies are born prematurely in the United States each year -- that's about one every minute -- causing an enormous toll in lost lives, lifelong health care challenges and high medical expenses.

And even though most of those babies are white, the rate of premature birth among African-American women is nearly twice as high as for whites and Hispanics, and premature black babies die more often than white ones, says Hyagriv Simhan, a researcher and prematurity expert at the Magee-Womens Research Institute.

While some may believe that those figures simply signal greater poverty and poorer prenatal care for black women, Dr. Simhan said the numbers prove otherwise.

Studies at his institute have shown that the higher the proportion of West African ancestry that a woman and her partner have, the higher the odds of premature birth. The statistics also show that an African-American woman in graduate school has a greater chance of giving birth prematurely than a white woman who didn't finish high school.

So what is going on?

That's what Dr. Simhan, director of the Center for Prematurity at Magee-Womens Hospital of UPMC, has dedicated himself to finding out.

While the evidence is indirect, he suspects that African-American women have a genetic vulnerability to premature birth that may intersect with something in the environment, including their diets or infections.

Research shows that many of the most dangerous premature births, those that occur before 34 weeks' gestation, are triggered by inflammation in the uterus, possibly from bacterial infections that move up the birth canal from the vagina. Once the infection reaches those tissues, the body's own immune response can trigger early labor, he said.

Black women not only get vaginal infections more often than other groups, Dr. Simhan said, but have more trouble containing them.

Another possible culprit for the higher rate of premature birth is a shortage of vitamin D, he said.

People with darker skin need more sunlight than their lighter-skinned counterparts to produce the same amount of vitamin D. While the sunlight that black Americans' ancestors received in West Africa would have been more than sufficient, the sunlight they get in much of North America may not be, he said.

A recent small study in South Carolina showed that vitamin D supplements improved pregnancy outcomes for black women, and Dr. Simhan is planning a large, multicenter trial to explore the possible benefits of supplements in preventing premature births.

Dr. Simhan and his colleagues around the country are at the forefront of a war that exacts extremely high costs in both dollars and lives.

A premature birth costs 15 times more than a routine one, he said, and that's just the direct doctor and hospital costs. It makes labor and delivery the fifth most expensive hospital cost in the country, he said. The other four are all related to heart disease.

Even if a premature baby survives the first weeks of life, there can then be enormous lifelong costs to the family and society, from screening and intervention for medical problems to special schooling. In fact, premature babies who are small for their gestational age will often then go through a catchup growth spurt that can set them up for obesity and high blood pressure later in life, he said.

While premature babies in industrialized nations are surviving more than ever before, early birth is still a killer in much of the world and causes the death of an estimated 1 million infants a year, according to the March of Dimes.

That's more than the number of American soldiers killed in the Civil War, leading Dr. Simhan to conclude that "it's appropriate to have a war on prematurity. As a condition it's kind of underrecognized because there is this cultural view of pregnancy as a happy time of expectation and there is not a lot of sense of the impact of prematurity unless you or your family has a premature baby."

It's not a conflict Dr. Simhan ever imagined signing up for.

Born in India, he moved to the Philadelphia area with his parents when he was 3.

When he was in medical school in Boston, obstetrics and gynecology was "the one thing I would have told you I never wanted to do." He was set on doing heart transplants.

But then he interned in ob-gyn at a city hospital, and he was soon hooked, not so much from the joy of delivering babies, but from "the notion of having two patients [the mother and fetus] and finding that the clinical and ethical and scientific aspects were incredibly interesting to me."

One of the most fascinating mysteries, Dr. Simhan said, is that the mother's body -- the same body that would be primed to reject a transplanted organ -- is able to nurture another creature inside it that has half its genes from another human being.

It's a process that is still not fully understood, he said, but seems to involve a major adaptation in the mother's immune system against foreign tissue markers.

Eventually, Dr. Simhan said, he wants to find out how to screen for prematurity risks and devise interventions much earlier in the birth process than now.

Many doctors focus on trying to halt premature labor after it starts, "but I think the predisposition to this happens weeks or months beforehand. The process is kind of like a heart attack, with a long buildup."

"The developing wisdom now," he said, "is that you have to look much earlier for these events."

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