The U.S. rate of pre-term births has fallen for the second year in a row, U.S. researchers reported on Tuesday.

The findings are good news, as babies born too early and too small are sicker and more likely to die than babies born after a full 39 weeks of gestation.

But pre-term birth rates remain high compared to other developed countries and doctors and officials still must work harder to figure out why so many American babies are born too soon, Joyce Martin of the National Center for Health Statistics and colleagues said.

“Following a long period of fairly steady increase, the U.S. pre-term birth rate declined for the second straight year in 2008 to 12.3 percent, from 12.8 percent in 2006,” they wrote in their report.

“This marks the first two-year decline in the pre-term birth rate in nearly three decades,” they added.

The March of Dimes, a charity founded to battle birth defects, said that still means 500,000 babies are born pre-term in the United States.

“Pre-term birth, birth before 37 weeks gestation, is a serious health problem that costs the U.S. more than $26 billion annually,” the group said in a statement.

“It is a leading cause of infant death, and babies who survive an early birth often face the risk of lifetime health challenges, including breathing problems, cerebral palsy, mental retardation and others.”

A major cause of pre-term birth is multiple pregnancy - twins, triplets and especially larger groups such as quadruplets are much more likely to be delivered early and to have health problems.

But poor prenatal care, poor nutrition and other factors also play a role, doctors say.

“In every state, our volunteers are working with policy makers to improve the quality of perinatal care, and determine best practices for reducing pre-term birth,” said Dr. Jennifer Howse, president of the March of Dimes. “We are thrilled with this sign of sustained progress.”

Last year, the NCHS reported that the United States ranks 30th in terms of infant mortality compared to other countries. It found that 1 in 8 births in the United States were born pre-term, compared with 1 in 18 births in Ireland and Finland.

Public health experts look at infant mortality in calculating the quality of a country’s healthcare system. The United States is often ranked behind other industrialized countries, in part because of the infant mortality rates, which are in turn linked to pre-term births.

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WASHINGTON (Reuters)