Australian scientists found that smoking by mothers has replaced infants sleeping on their stomachs as the greatest modifiable risk factor for unexpected infant death syndrome, or SIDS.

They found that when mothers smoke, the sleep arousal the process of infants, which awakens them in response to a life-threatening situation, is altered, increasing the risk for SIDS.
Researchers studied 12 healthy, full-term infants born to mothers who smoked cigarettes an average of 15 cigarettes a day. The infants' arousal responses during daytime sleep were compared with those of 13 healthy infants of nonsmoking mothers. All the children were assessed three times: at 2 to 4 weeks of age, 2 to 3 months, and 5 to 6 months.
At the end of the investigation they found that infants who had been exposed to smoke had reduced sub-cortical activation to cortical arousal. They also had lower rates of full cortical arousals from sleep and higher rates of sub-cortical activations than infants of nonsmoking mothers.
The study also identified a dose-dependent relationship between cortical activation rates and levels of infant urinary cotinine, a nicotine metabolite. Infants with the highest levels of smoke exposure had the lowest levels of cortical arousal.
They also noted that decreased cortical arousals from sleep have been observed in infants who later died of SIDS.
This very important study showed that maternal smoking can impair the arousal pathways of apparently normal infants, which may explain their increased risk for SIDS.
Although sudden infant death syndrome cannot be prevented, there are several factors which are known to be associated with increased risk. For example:

• Smoking during and/or after pregnancy:
• In an American study, of all SIDS cases, 21% were attributable to maternal smoking whilst among smokers, 61% of SIDS cases were attributable to maternal smoking.
• Maternal smoking during pregnancy is associated with a significantly increased risk of SIDS.
• There is even a relationship between maternal smoking and changes in the brainstem of the infant.
• Low pregnancy weight gain
• Age less than 20 at first pregnancy
• Placental abnormalities
• Alcohol and substance abuse
• History of sexually transmitted disease.