Which of the following is NOT a known risk factor of SIDS?

Putting a baby to sleep on the back is widely recognized as a protective factor against Sudden Infant Death Syndrome (SIDS), rather than a risk factor. Research has shown that infants placed on their backs for sleep have a significantly reduced risk of SIDS compared to those who sleep on their stomachs or sides. This recommendation is part of the "Back to Sleep" campaign implemented in the early 1990s, which helped decrease the incidence of SIDS.

In contrast, the other options listed are well-established risk factors. Sleeping on the stomach is known to increase the chances of SIDS because this position can affect how an infant's airway functions. Parental smoking is also linked to a heightened risk, as it can lead to respiratory issues and decrease the overall health of the infant. Lastly, sleeping in a crib with loose bedding poses a suffocation risk and can increase the likelihood of rebreathing carbon dioxide, both of which are associated with SIDS. Thus, the choice of putting a baby to sleep on the back is not only safe but actively recommended to help prevent SIDS.