The death rate of U.S. newborns during the first week of life has decreased by 8 percent in recent years, but the rate of stillbirths late in pregnancy has held steady, according to a new report from the Centers for Disease Control and Prevention (CDC).

The report will help researchers understand the health of pregnant women in the United States, said lead researcher Elizabeth Gregory, a health statistician with the CDC's National Center for Health Statistics.

For the report, researchers looked for changes between 2006 and 2012 in the country's "perinatal mortality rate," which includes the mortality rate of fetuses during the last trimester of pregnancy as well as that of newborns during the first week after birth.

They found that the overall U.S. perinatal mortality rate decreased by 4 percent over the study period, according to the report. Perinatal rates declined in 14 states, remained unchanged in 35 states and rose in only one — South Dakota — between 2006 and 2012, according to the report.

This decline in the overall rate of fetal and infant deaths is noteworthy, the researchers said.

But when the researchers focused on the rate of deaths during the last trimester of pregnancy, they found the rate stayed relatively flat during the study period, the report said. Previously, the rate of fetal deaths had been declining — it fell by 8 percent between 2000 and 2006. 

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In contrast to the steady rate of fetal deaths, the death rate of newborns younger than 7 days old fell, and the drop was most pronounced among babies born to black women. The infant mortality rate for this group dropped by 8 percent between 2006 and 2011, according to the report. The mortality rates of infants born to white and Hispanic women did not change substantially during the study period, the report said.

It's unclear why more newborns are surviving, but between 2006 and 2012, preterm birth rates fell, and the percentage of babies born at 39 weeks of pregnancy or more increased, the researchers said. Studies show that longer pregnancies reduce newborn death risk.

Until recently, preterm births in the United States had been increasing, said Dr. Eve Lackritz, deputy director of the Global Alliance to Prevent Prematurity and Stillbirth at Seattle Children's Hospital. In response, doctors began a nationwide effort to reduce inductions and cesarean deliveries before 39 weeks, unless they were medically necessary.

It's possible that efforts to decrease preterm births are linked to the drop in newborn mortality, although research is needed to determine if they are related, said Lackritz, who was not involved in the new report.

Better neonatal care has helped struggling newborns survive across the country, Lackritz told Live Science. The drop in the death rate of newborns born to black women is "exciting news, and an important indicator for the United States in making gains in narrowing racial disparities in infant mortality," Lackritz said.

However, "critical racial disparities continue to plague progress in infant mortality in the United States," she said.

In 2012, there were 3.95 million live births in the U.S., Gregory said.

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