- CDC data show state-by-state and racial gaps remain in America's infant mortality rate
- Mississippi had highest rate, Massachusetts had lowest from 2013 through 2015
(CNN)There's good and bad news when it comes to infant mortality in the United States.
Infant mortality, or the death of a baby before his or her 1st birthday, has declined in recent years across the nation and around the world.
Yet disparities persist across states, according to a report released by the Centers for Disease Control and Prevention's National Center for Health Statistics on Thursday.
The new report showed that, from 2013 through 2015, the infant mortality rate in the US ranged from 9.08 deaths per 1,000 infants born alive in Mississippi -- which had the highest rate -- to 4.28 deaths per 1,000 live births in Massachusetts, which had the lowest.
A report released last year showed that the overall infant mortality rate in the US dropped 15% from 2005 to 2014.
The new report includes data for 2015, showing that the overall rate remained similar, declining 14% from 6.86 deaths per 1,000 live births in 2005 to 5.9 deaths per 1,000 in 2015.
The report involved data from the National Vital Statistics System, including birth and death certificates from 2013 through 2015 for infants under 1 year old.
"We did something a little bit different this time, and that was to look at the state rates by race and ethnicity," said T.J. Mathews, lead author of the report and a demographer at the CDC's National Center for Health Statistics.
Between 2013 and 2015, the mortality rate among the infants of non-Hispanic white women ranged from a low of 2.52 deaths per 1,000 live births in the District of Columbia to a high of 7.04 in Arkansas, according to the report. The lowest mortality rates among this group occurred mostly in the western and northeastern regions of the country.
The rate among infants of non-Hispanic black women ranged from 8.27 in Massachusetts to 14.28 in Wisconsin, according to the report. Five of the nine highest mortality rates for infants of black women were in Midwestern states.
Among infants of Hispanic women, the rate ranged from 3.94 in Iowa to 7.28 in Michigan.
Of the three groups presented in the report, infants of white women had the widest range in mortality rates by state, with the highest rate being 2.8 times the lowest.
Still, "the lowest rate for non-Hispanic black is higher than the highest for non-Hispanic white or for Hispanics," Mathews said. "For people who don't necessarily think about this topic, this says a lot."
The new CDC report did not investigate why such regional and racial differences exist, but some studies and experts have explored answers to that question.
The five leading causes of infant deaths in the United States in 2015 were birth defects; being born too early or having low birth weight; sudden infant death syndrome, known as SIDS; maternal pregnancy complications; and injuries, such as suffocation, according to the CDC.
"We have to really take a broader look at what are the social and structural conditions in this country that lead to high rates of infant mortality, but also that lead to these unacceptable disparities in infant mortality," said Dr. Paul Jarris, chief medical officer for the March of Dimes, a nonprofit focused on the health of mothers and babies. He was not involved in the new CDC report.
"We know there are differences in rates of premature birth and maternal mortality and infant mortality based upon social factors, such as educational level, income level, housing safety, safeness of a neighborhood, employment -- so factors like this make a big difference," Jarris said.
Similar infant mortality disparities emerged in a 2016 paper that Alice Chen, an assistant professor of public policy at the University of Southern California, co-authored with Emily Oster, a professor of economics at Brown University, and Heidi Williams, an economist at the Massachusetts Institute of Technology.
In the paper, published in the American Economic Journal: Economic Policy, "we find that 45% of regional differences can be attributed to differences in birth weight, with lower birth weights in states like Oklahoma, Arkansas, Louisiana, and Texas, especially relative to the Northeast," said Chen, who was not involved in the new CDC report.
"This means that after adjusting for birth weight -- comparing infants of the same birth weight across states -- large geographic disparities persist," she said.
Chen added that the majority of the racial gap in infant mortality also can be explained by lower birth weight and earlier-term births.
"The remaining differences are due mainly, 67%, to deaths in the postneonatal period, from 1 month to 1 year of life, when infants are out of the hospital system, as opposed to the neonatal period, with survival less than one month," she said.
Overall, a major cause of the recent decline in the nationwide infant mortality rate in the United States has been the widespread use of safe sleep practices in households with infants, Jarris said.
"Do they sleep in their own space? Do they sleep on their backs? Are they free of things like lots of blankets, pillows or bumpers that they could asphyxiate on?" Jarris said.
"If a mom lives in a home where she is unable to afford a crib or unable to obtain a crib, and therefore may have to keep the baby in her own bed, that is something that is highly associated with infant mortality," he said. "What it means is, we really can't separate out infant mortality from the conditions in which people live in this country. It's not simply a clinical or research question. It's a social justice question, if you will, for our nation."