More than half a million babies are born prematurely each year, and specialists are urging that doctors take new steps to battle one cause: infertility treatments that spur twins, triplets and other multiple births.

But despite a booming business, infertility treatment explains only a fraction of the nation's huge and growing problem of prematurity. One in eight babies now is born at least three weeks early, many even earlier, a rate that has increased more than 30 percent in two decades.

Trying to help these fragile infants survive and thrive costs the nation at least $26 billion a year, and there's little likelihood of improvement soon, says a sobering report released Thursday by the Institute of Medicine.

That's because doctors don't know the cause for most preterm births or how to prevent them, and have few good ways even to predict which women will go into preterm labor, concludes the report, which calls for urgent research to try to turn the tide.

"We don't have a good handle on prematurity," says report co-author Dr. Marie McCormick of Harvard University, who receives anguished phone calls from new mothers asking, "I did everything right, why did I have a premature baby?"

McCormick wants women to know: "If she delivers prematurely, don't think she's done something wrong."

Babies born before 37 weeks of pregnancy are premature. Those born before 32 weeks face the greatest risks of death ¡ª about one-fifth don't survive a year ¡ª and long-term health problems, such as cerebral palsy, retardation or learning disabilities, asthma and other conditions.

Doctors have made great strides in helping preemies to survive, even those born as young as 23 weeks, and most who do survive infancy grow up fairly healthy. But being even a few weeks premature can increase the risk of health and developmental problems.

Any woman could have a premature baby. But black women have the highest risk: 17.8 percent of their babies are born prematurely, compared with 11.5 percent of white women and 11.9 percent of Hispanic women, the report found.

Poor women are more at risk, too, as are mothers-to-be who are under age 16 or over 35. Certain infections can trigger preterm labor. Other risk factors include poor diet, maternal stress, lack of prenatal care and smoking.

But differences in behavior and socio-economic conditions can't fully explain the disparities, the institute cautioned.

In fact, the prematurity rate for black women has slightly improved in the last decade even as it increased among white women. Why? Black women are less likely to undergo the infertility treatments increasingly embraced by white women, McCormick said.