Babies should not be left asleep in car seats unchecked because there is a risk they will stop breathing, warn experts.
It is a legal requirement for children travelling in a car to use a car seat
Problems arise because reflexes that keep the baby's head upright and their breathing normal are still developing.
The British Medical Journal study authors say modifying car safety seats so a baby's head does not flop forward could help avoid the risk.
As well as being used for car transport the seats are often used to carry babies around outside and in the home.
But allowing a baby to fall asleep restrained in a relatively upright position could be lethal, the New Zealand researchers warn.
They looked at nine babies who were referred to the Auckland Cot Monitoring Service between July 1999 and December 2000 after an apparently life-threatening event.
All nine were described as "blue", "scrunched up" and "not breathing".
Eight occurred while the infant was sleeping in a car safety seat in the home, and only one while the baby was in a seat in the car while being driven.
The babies, aged three days to six months, had been otherwise healthy and, at the time, had been slumbering in a car seat appropriate for their age.
The investigators reconstructed the scene, using the infants' own car seat, to establish the precise position of each baby when the event occurred.
They found the babies' heads tended to loll forwards, pressing the jaw into the chest, which made it harder for them to breathe.
Also, the muscles of the throat tend to relax when a person is asleep, narrowing the airway further.
But the researchers say other factors may also be important.
Half the mothers in the study were smokers, and smoke exposure is known to increase the risk of cot death.
The team gave all of the infants breathing monitors and the parents advice on appropriate positioning, including not leaving the infant for excessive periods in the car seat.
None of the infants had any further reported problems over the next 12 months.
Past work suggests newborn and particularly premature babies can develop breathing problems from sitting for too long in car seats.
The Foundation for the Study of Infant Deaths advised parents that the safest way for a baby to sleep was flat on their back and in their own cot or crib, preferably in the same room as a parent or other carer.
A spokeswoman said: "Car seats are not ideal places for safe infant sleep in the home.
"If a baby is being transported in a car, he or she should be carried in a properly designed and fitted car seat, carried backwards and be observable by a competent adult."
A spokesman from the Royal Society for the Prevention of Accidents said: "Child car seats are improving in design all the time and new research is always welcome if it can help improve the design further."
He stressed that the numbers in the study were very small and the chance of any breathless event was rare.
"We would hope that nothing like this would stop people from using child car seats in their car. Not only is it a legal requirement, they save lives."