(NewsNation Now) — A study of sudden unexpected infant deaths, or SUID, from 2011-2017 found 72% occurred in an unsafe sleeping environment.

The study, published Tuesday in the Official Journal of the American Academy of Pediatrics, looked at more than 12,000 cases of SUID. The American Academy of Pediatrics says it’s the leading cause of death for infants between a month and a year old.

The study found that 75% of airway obstruction deaths were tied to soft bedding. Not all cases had a definitive cause of death.

SUID is an umbrella term for three types of infant deaths: sudden infant death syndrome, accidental suffocation and strangulation in bed, and deaths from unknown causes. The CDC reports there are about 3,500 cases per year in all the categories combined.

The AAP recommends keeping infants on their back on firm mattresses in otherwise empty cribs. The children should not sleep on sofas, armchairs, or adult beds. They say breastfeeding also could reduce the risk of SUID.

A previous study found that infants placed on their stomachs had a nearly two-fold increased risk of dying from SUID.

Alabama, Arkansas, and Mississippi had the three worst SUID rates among states from 2014-2018. Alabama reports 178.3 SUID victims per live 100,000 births over that time.

“Further analysis of unexplained deaths, including continued improvements to death scene investigation and documentation, may generate hypotheses for physiologic and genetic research, advance our understanding of gaps in SUID investigation, and enhance our understanding of infants at highest risk,” the new study’s conclusion reads.