About 1 in 8 U.S. births are premature. Early births are much less common in most of Europe; for example, only 1 in 18 babies are premature in Ireland and Finland.

Poor access to prenatal care, maternal obesity and smoking, too-early cesarean sections and induced labor and fertility treatments are among the reasons for preterm births, experts said.

Premature babies born before 37 weeks tend to be more fragile and have under-developed lungs, said the lead author of the new report, Marian MacDorman of the U.S. Centers for Disease Control and Prevention.

Premature births are the chief reason the U.S. ranks 30th in the world in infant mortality, with a rate more than twice as high as infant mortality rates in Sweden, Japan, Finland, Norway and the Czech Republic. For several years, the U.S. has ranked poorly among industrialized nations. MacDorman's report scrutinizes the reasons for that.

If U.S. infants were as mature as Sweden's are at birth, nearly 8,000 premature infant births could be avoided and the U.S. infant mortality rate would be about one-third lower than it is, according to a calculation by MacDorman and others at the CDC's National Center for Health Statistics.

Why so many more premature infants here? Experts offered several possible explanations:

_Fertility treatments and other forms of assisted reproduction probably play a role because they often lead to twins, triplets or other multiple births. Those children tend to be delivered early.

_The U.S. health care system doesn't guarantees prenatal care to pregnant women, particularly the uninsured, said Dr. Alan R. Fleischman, medical director for the March of Dimes.

_Maternal obesity and smoking have been linked to premature births and may also be a factor.

_Health officials are also concerned that doctors increasingly are inducing labor or performing C-sections before the 37th week. However, Fleischman said most infant deaths do not occur in babies just shy of 37 weeks gestation, but rather in those much younger,

Labor was induced in nearly 16 percent of premature births in 2006, up from about 8 percent in 1991. Cesarean sections were done in 36 percent of preterm births, up from 25 percent in 1991, MacDorman said.

The report used 2005 statistics to make comparisons to 14 European countries. There is more recent data: International infant mortality statistics for 2006 and 2007 indicate that since 2000, the U.S. rate has stood at about 7 infant deaths for every 1,000 live births.

The report also found that while the United States more commonly saw premature births, survival rates for infants at that gestational age were as good or better than most European countries.

"So, once the baby is born too early, we do a good job of saving it. What we have trouble with is preventing the preterm birth in the first place," MacDorman said.

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On the Net:

http://www.cdc.gov/nchs