At one time, researchers thought letting babies sleep on their stomachs was the greatest modifiable risk factor for sudden infant death syndrome (SIDS), but now they say it could be mothers who smoke. The smoke may affect a baby’s ability to respond to life-threatening situations, which raises his or her risk for SIDS.

Researchers from the Ritchie Centre for Baby Health Research at Monash University in Melbourne followed 12 healthy infants born to mothers who smoked at least 15 cigarettes daily. The babies’ arousal responses—controlled by the part of the brain that increases wakefulness, vigilance, muscle tone, heart rate and breathing rate—were compared with those of 13 healthy babies born to non-smoking mothers. They found that the infants who were exposed to smoke had lower rates of full cortical arousals than the babies who weren’t exposed to smoke. Researchers also observed low cortical arousal in infants who ultimately died of SIDS.

According to the American Lung Association, SIDS is the third leading cause of death for African-American babies, who are 2.5 times more likely to die of SIDS than white babies.

Learn more about SIDS at