The researchers compared data on infant health and mortality in the U.S.; Austria, whose rate of 3.8 is roughly average among European nations; and Finland, whose rate of 2.3 is one of the lowest in the world. One of the biggest differences, they found, was in the definition of what could be considered a live birth. “Extremely preterm births recorded in some places may be considered a miscarriage or still birth in other countries,” they wrote. Although the chance of survival for babies born before 23 weeks is low (the American Academy of Pediatrics recommends that doctors don’t resuscitate babies born before that point), they’re recorded as live births in the U.S.

“There’s a viability threshold—we basically have never been successful at saving an infant before 22 weeks of gestation,” says Emily Oster, a professor of economics at the University of Chicago and one of the study authors. “When you do comparisons, if other countries are never reporting births before that threshold as live births, that will overstate the U.S. number relative to those other places, because the U.S. is including a lot of the infants who presumably existed as live births.”

This difference in reporting, they found, accounted for around 40 percent of the U.S.’s relatively high rate compared to Austria and Finland, a result supported by the CDC report—when analysts excluded babies born before 24 weeks, the number of U.S. deaths dropped to 4.2 per 1,000 live births.

While this may account for a significant chunk of the difference, though, it doesn’t explain it away entirely—especially because much of the high U.S. rate can be attributed to babies who die months after they leave the hospital. When the researchers broke the statistics down by age, they discovered that neonatal deaths were actually less frequent in the U.S. than in Austria and Finland. (“Neonatal” refers to infants up to a month old, while “postneonatal” includes those between one month and one year.) In other words, American babies are mostly fine while they’re in the hospital and during their first days at home—but over time, that changes.

Or rather, it changes for some of them. The effects of socioeconomic status on health have been well-documented, and infant mortality is no exception: Unsurprisingly, the states with the highest rates are also among the poorest. “If Alabama were a country, its rate of 8.7 infant deaths per 1,000 would place it slightly behind Lebanon in the world rankings,” Christopher Ingraham recently noted in The Washington Post, while “Mississippi, with its 9.6 deaths, would be somewhere between Botswana and Bahrain.”

When the researchers took socioeconomic status into account, they found no significant difference in mortality across the three countries among babies born to wealthy, well-educated women. Lower down the socioeconomic ladder, though, the differences became stark; children of poor minority women in the U.S. were much more likely to die within their first year than children born to similar mothers in other countries.

“I don’t think we have a deep understanding of what’s going on there,” Oster admits.

But, she adds, the data can be useful even without the knowledge of why it looks the way it does. “If you want to think of this from a policy standpoint, [this information] would be an important start,” she says. “The kinds of interventions you’d want to engage in would be very different if you thought the problem was entirely with death in the first month.”

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