More babies are dying before they turn 1 year old in the United States than in most of Europe and several other developed countries, a new U.S. government report says.

A greater proportion of premature births and deaths of full-term babies are driving the higher rate, which puts the United States below 25 other countries, according to the report, released Sept. 24 by the Centers for Disease Control and Prevention.

"I think we've known for a long time that the U.S. has a higher preterm birth rate, but this higher infant mortality rate for full-term, big babies who should have really good survival prospects is not what we expected," said lead author Marian MacDorman, a senior statistician and researcher in the CDC's National Center for Health Statistics.

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The infant mortality rate refers to the percentage of babies born alive who die before their first birthday.

The report compares infant mortality rates in the United States to those of European countries plus Australia, Israel, Japan, Korea and New Zealand in 2010, the most recent year for which data is available.

In the United States, 6.1 infants die per every 1,000 live births, more than double the rates of Finland, Japan, Portugal, Sweden, the Czech Republic and Norway. Still, the U.S. rate is an improvement since 2005, when the rate was 6.87 and had not budged much for five years, according to the report.

Infant Mortality Rates, 2010

Source: CDC

The reasons for the higher U.S. rate are complex. Improvements in prenatal care have led to more preemies, who are at higher risk for death, instead of stillbirths, said Dr. Deborah Campbell, a professor of clinical pediatrics at Albert Einstein College of Medicine in New York City. Stillbirths are not included in infant mortality statistics.

But disparities in prenatal care play a role as well, she said.

"The U.S. lags behind other developed countries because there remain significant gaps in access to and utilization of prenatal and preconception care," said Campbell, who is also director of division of neonatology at Children's Hospital at Montefiore Medical Center in New York City. "There is a well-delineated history of racial and ethnic disparities in maternal and infant outcomes in the U.S., with black women and their infants being at greatest risk and having the highest rates of poor outcomes," she said.

Campbell added that it can take three to four generations to overcome effects from past health problems such as malnutrition, discrimination and lack of access to quality health care.

At the same time, it appears that more babies are born before 24 weeks in the United States than in other countries, MacDorman said.

"I think that we make greater efforts to save really tiny babies at 22 or 23 weeks than in some other countries," she said, although she added that the report didn't address the reasons for her team's findings.

"Whether that's a good thing or a bad thing, I think depends," MacDorman said. "We might have more of those tiny babies surviving, but they might survive with terrible disabilities, long-term health problems and so forth."

When her study team excluded births before 24 weeks, the U.S. rate improved to 4.2 deaths per 1,000 live births but still lagged behind nine other countries and remained about double that of Denmark, Finland and Sweden.

The infant mortality rate specifically among early preemies (24 to 31 weeks) was mostly similar in the United States and Europe, but the U.S. rate for babies born between 32 and 36 weeks was poorer. For babies born at 37 weeks or later, the United States ranked last.

"These are full-term babies who presumably are pretty healthy," MacDorman said. "This report doesn't directly describe what's going on there, but I think it's more about social factors, such as SIDS [sudden infant death syndrome] and injuries. I don't think it's so much about health care but about the environment and raising a child."

According to Campbell, risks for these babies include birth defects and chromosomal abnormalities, low birth weight, SIDS, injuries like accidental suffocation and conditions in mothers that also affect newborns, such as gestational diabetes or pre-eclampsia.

Several of these conditions occur more often among black and Native American/Alaskan Native women, Campbell said.

She noted, however, that several U.S. agencies and organizations began a push in 2009 to improve the preterm birth rate and prenatal care. One large part of this effort included eliminating non-medically necessary deliveries before 39 weeks of pregnancy.

"This has dramatically reduced the number of infants born late preterm," Campbell said. Another improvement has included promoting standardized, evidence-based practices in prenatal and newborn care, she said.

The researchers estimated that if the United States were to match Sweden's rates of premature births and deaths of full-term babies, 7,300 fewer infant deaths would occur each year, based on the 2010 figures.