Auckland University researchers believe they have solved the mystery of why sudden unexpected deaths of babies peak at the age of 2 months.
Senior researcher Dr Shirley Tonkin and colleagues say it is at least partly because of the different timing of two developments: the tongue grows at around two months, while it is not until six months that the jaw joint becomes stable.
Some babies are therefore temporarily vulnerable to having a relatively small airway gap around their tongue. This happens when their chin is pushed back, causing the tongue to obstruct the airway.
Flexing a young baby's head forward pushes the chin backwards. Dangerous chin pressure can occur in a baby sling or car seat, from being put to sleep on the tummy or side, or even from a sleeping baby rolling against someone else if they are sharing a bed, or having the person roll against them.
Every year 50 to 60 babies die unexpectedly. The median age of these deaths is 11 weeks.
Dr Tonkin said yesterday the latest findings, made with Professor Alistair Gunn and Dr Christine McIntosh, stemmed from a re-analysis of x-rays of 17 babies in car seats which had led to the development of foam inserts.
These prevented the baby's head slumping forward into a position that caused a reduction in oxygen levels.
Dr Tonkin said the x-rays of babies with the head flopped forward showed air was flowing through the upper airway of those who had a small tongue, but not those with a large tongue.
"The offshoot of this is that it's not entirely the mother's fault if something happens.
"Some babies are predisposed to having their airway closed off if they have a big tongue when they're born. Others are less likely to have anything go wrong."
She said sleeping parents should not have a baby in bed with them, because of the risk.
To explain this to parents of babies, the Cure Kids charity has financed a television advertisement, to air from next week. It will promote the safety message of putting babies to sleep in their own cradle, near the mother's bed.
So-called pepi-pods - and a flax version - are available to put a baby to sleep in its own cradle-like space on top of its mother's bed, if there's room. Dr Tonkin said this method was being tested to see if it was safe.
* Sudden unexpected death of an infant peaks at age 2 months.
* Tongue grows before jaw joint has firmed up.
* Pressure on chin - danger.
Ministry of Health advice to reduce risk:
* No smoking during pregnancy.
* Put baby to sleep on back.
* Baby should sleep in bassinet or cot in parents' bedroom.