The U.S. infant mortality rate is more than double that of some other developed countries, according to a new report.

An important contributor to this disparity is the relatively high rate of death among babies born at full term in the United States, compared with that of other countries, the report from the Centers for Disease Control and Prevention found.

Dr. Edward McCabe, chief medical officer of the March of Dimes, a nonprofit organization that works to prevent preterm birth, said he was surprised by this result. The findings "challenge the conventional thinking" that America's high infant mortality rate, relative to other countries, is mainly due to high rates of preterm births in the United States, McCabe said.

The report compared the U.S. infant mortality rate with that of 28 other developed countries. The CDC defines infant mortality as the death of a baby before his or her first birthday.

In 2010, there were 6.1 deaths for every 1,000 live births in the United States, which was higher than the rates of 25 other countries in the report, including Hungary, Poland, the United Kingdom and Australia.

In the top-ranked countries, Finland and Japan, the infant mortality rate was 2.3 deaths per 1,000 live births — less than half the rate in the United States. [7 Facts About Home Births]

Despite improvements in the U.S. infant mortality rate since 2005, "This pattern of high infant mortality rates in the United States when compared with other developed countries has persisted for many years," the researchers at the CDC's National Center for Health Statistics wrote in the report.

In a second analysis, the researchers parsed out the infant mortality rates according to babies' gestational age (meaning how long the baby was in the womb before birth), for the United States and 11 other European countries that had this information.

The U.S. mortality rate for infants born very early, between 24 and 27 weeks of gestation, was favorable compared with the other countries — the U.S. ranked 5th out of the 12 countries. (The researchers excluded babies born before 24 weeks of pregnancy, because not all of the countries had information about this group.)

In contrast, the U.S. infant mortality rate for babies born at 37 weeks or later (considered "full term") was actually the highest among the 12 countries, and about twice the rates in Denmark, Finland, Norway, Sweden and Switzerland.

The reasons for the findings are not known, but the new report brings attention to the problem, McCabe said.

"People are going to want to try to get to the bottom of it and understand it better," so that action can be taken to further reduce infant mortality, McCabe said.

An earlier analysis by the March of Dimes found that most deaths among babies born at full term were due to birth defects, sudden infant death syndrome and accidents (such as accidental drowning).

If the United States could reduce its infant mortality rate for full-term babies to match that of Sweden, the overall U.S. infant mortality rate would decline by 24 percent, to 3.2 deaths per 1,000 live births, the report said. This would mean there would be nearly 4,100 fewer infant deaths yearly.

If the United States reduced its preterm birth rate to match that of Sweden, that would mean an additional 3,200 fewer deaths.

"Such a decline would mean nearly 7,300 fewer infant deaths than actually occurred in the United States in 2010," the researchers wrote.

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