Why we lay babies on their backs:

Research suggests that healthy newborn infants do not have what doctors call "nasoaxillary reflex" -- a protective reflex that helps keep their nasal passages open.

In adults lying on their side, the nasoaxillary reflex ensures that the uppermost nasal airway is open, Dr. Christopher O'Callaghan of the University of Leicester, UK, and colleagues explain in the journal Archives of Diseases of Childhood.

The researchers used acoustic rhinometry, a technique that measures nasal patency, to see whether the nasoaxillary reflex is present in 11 healthy term newborns.

Acoustic rhinometry emits wide band noise into the nose and analyzes the reflected sound in order to measure cross sectional area/distance mapping of the nasal cavity. The measurements were made while the infants were lying on their back (the supine position) and on their side (the lateral position).

The investigators were unable to show a protective nasoaxillary reflex in the infants.

The absence of the reflex in infants is not all that surprising. Actually babies lack a lot of the reflexes that mature adults have, and have a couple that adults don't have -- like reflexively grabbing your finger when you put it in their hand. This is not because they lack pathways that develop later in life. Actually, they have the pathways already. Rather, the pathways haven't myelinated yet. Myelination actually proceeds throughout life, and numerous developmental and cognitive milestones can be related to the onset of myelination in a particular brain area.

Anyway, if this explains part of why SIDS deaths occurs, it would be certainly good to know. We already know that putting babies on their sides slightly increases SIDS, so this is just one more reason to put babies on their backs.