Former Registered Nurse,
now babywearing educator and mother of two
First, the design of these bag slings causes baby to curl chin to chest, larger babies more so because their heads are positioned further up in the carrier. This position kinks baby’s airway causing the baby to work harder to breathe.
For more information on the importance of maintaining an infant's head and neck in an aligned position see the articles:
“Baby car seat ‘cot death’ concerns”
“Simple Car Seat Insert to Prevent Upper Airway Narrowing”
PEDIATRICS Vol. 112 No. 4 October 2003, pp. 907-913
Third, it is difficult for the parent to closely monitor their infant unless he/she pulls open the top of the sling. Bag slings are generally deep, plus they sag when baby is placed in it, further increasing the depth of the carrier. The gathered top, and the fact that the sling hangs so low, obstructs the parent’s view of baby. If a newborn were to have difficulty breathing, and/or rotate until his nose and mouth was pressed against the side of the carrier, the parent may not be aware of the baby’s respiratory distress for some time. Compounding this problem is the difficulty of feeling the baby's distress through the thick fabric of the sling.
Fourth, although one bag sling is designed with large mesh panels placed near the infant’s head, others are not. There is a possibility that, with only a very small opening at the top of a non-mesh sling, an infant may not receive adequate amounts of fresh air. There is also the concern that carbon dioxide levels could rise the longer the infant remains in the sling.
For more information on the dangers of re-breathing see this article:
“About Carbon Dioxide Poisoning and SIDS”
Compare bag slings with a shallow fabric pouch or adjustable open-tailed sling (or mei tai or wrap). In these types of carriers an infant is easily monitored and visualized. Also, a newborn's head is effectively sandwiched between the sides of these carriers, preventing the infant from easily rotating his/her head into the sides of the carrier."
Please, please seriously consider before you purchase a 'bag sling'. The last I saw, these slings are promoted as "NEW!" and "MUST BUY!" in our Malaysian market.
Read more on the different tests being done on these slings. These are definitely worth a read especially if you are planning to buy a 'bag sling' (more so if you already own one)