That's more than 500,000 babies a year, a steadily rising number as the rate of premature birth has grown by more than 30 percent in two decades.
Helping 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 from the Institute of Medicine.
That's because doctors don't know the cause of 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.
"It can happen to anyone," warned report co-author Dr. Jay Iams of Ohio State University.
That is likely to come as a surprise to many pregnant women. Happy stories of the tiniest preemies who survive are common enough that there's a widespread false sense that modern medicine is conquering prematurity, Iams lamented.
"That translates into, 'It's no big deal to have my baby born early,"' he said. "Of course, it is a big deal. ... This is a problem that is not appreciated by the public in the United States, and that's a reason it has not received the kind of research support it deserves."
Scientifically, too, "there's just enormous ignorance" about even the basics of prematurity's causes, added Dr. Richard Behrman, a Menlo Park, Calif., pediatric specialist who chaired the institute's probe.
A full-term pregnancy lasts from 38 to 42 weeks. Babies born before completion of week 37 are premature. Those born before 32 weeks face the greatest risks of death — about one-fifth don't survive a year — and devastating disabilities such as cerebral palsy and retardation. But even babies born just a few weeks early can face developmental delays and other problems.
Who's at high risk?
—A previous preterm baby doubles the risk of a second.
—Carrying twins increases prematurity risk by 40 percent, odds that worsen with triplets or more.
—Some 17.8 percent of black women's babies are born prematurely, compared with 11.5 percent for white women and 11.9 percent for Hispanics. That troubling difference can't be fully explained by income, education or access to prenatal care.
—Women pregnant through certain infertility treatments have increased risk, mostly because they're more likely to carry twins or more. But recent research suggests even single babies conceived by in-vitro fertilization are more likely to be preterm.
—Poor women are more at risk, as are those who are under age 16 or over 35.
—Certain infections can trigger preterm labor, and other risk factors include poor diet, maternal stress, lack of prenatal care and smoking.
Still, even women with no obvious risk factors have preemies. Report co-author Dr. Marie McCormick of Harvard University wants new mothers to know: "If she delivers prematurely, don't think she's done something wrong."
The Institute of Medicine, independent advisers to the government, urged major new federal research into prematurity's causes and how to prevent it. Until then, it recommended that:
—More pregnant women receive a first-trimester ultrasound exam, the only way to be certain of the fetus' exact age. That's particularly important if the woman later has labor induced or a Caesarean section before her due date, either elective or because of a possible health problem.
—Specialists should strengthen guidelines that reduce the number of multiple births as a result of infertility treatment. During in-vitro fertilization, doctors often implant several embryos at once into a woman's womb. That number has been dropping thanks to guidelines from the American Society of Reproductive Medicine, issued in 1999 and tightened in 2004 — and triplet-and-higher multiple births have dropped, too.
Still, the group is considering tightening the guidelines further.
But it won't be easy to follow the example of some European countries that implant just one embryo at a time, said Dr. William Gibbons, president of the Society of Assisted Reproductive Technology. Those countries also pay for women to undergo multiple IVF attempts, while very few American women have insurance coverage for a procedure that can cost more than $15,000 per try, he noted.
"If we want to buy into this, society needs to buy into it," said Gibbons — who added that parts of Europe also found they saved money on treating preemies even after paying for repeated single-embryo IVF attempts.
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