U.S. public health officials have long puzzled over a troubling statistic about infant deaths: Why is the rate so much higher in rural counties than urban areas? But their understanding of the complex factors behind the disparity has been very limited.

A report released by the National Center for Health Statistics on Thursday provides important new information by delving into the five leading causes of infant death: congenital malformations, low birth weight, sudden infant death syndrome (SIDS), maternal complications and unintentional injuries. And it turns out that rural counties had higher mortality rates for some, but not all, of those causes.

Researchers looked at birth and death data from 2013 to 2015 and found that rural counties had the highest infant mortality rate at 6.69 deaths per 1,000 live births. Next came small and medium urban counties with 6.29 deaths per 1,000, and finally large urban counties at 5.49 deaths per 1,000. The pattern was not unexpected, based on previous data.

When the researchers looked more closely, they found that the rate was highest in rural counties for three of the five leading causes of death: congenital malformations, SIDS and unintentional injuries. But it was lower for the other two causes: low birth weight and maternal complications.

For post-neonatal deaths — those occurring between 28 and 364 days of age — the mortality rate was highest in rural counties for four of the five leading causes: SIDS, congenital malformations, unintentional injuries and homicides. With SIDS, the rural rate was almost double the urban rate — 5.5 vs. 2.78 per 1,000 live births.

Only for deaths caused by diseases of the circulatory system did rural counties not have the highest rate, trailing small and medium urban counties.

While the purpose of the report was simply to document the numbers, study authors Danielle Ely and Donna Hoyert theorized that some of the gap “could be related to differences in conditions during pregnancy.” Past research has shown that poverty, smoking and other maternal health behaviors during pregnancy and less access to care all can impact infant mortality.

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