Parents should not leave babies sleeping unattended in standard car safety seats because it may trigger serious breathing problems, a new study suggests.

Researchers at the University of Auckland in New Zealand studied 43 reports of babies who suffered a lack of oxygen — nine had been left to sleep in their car seats.

Parents described their children, who had been left to rest in the car seats, as "blue," "scrunched up" and "not breathing." The babies, who ranged in age from three days to six months, all recovered after the event.

The study, published in this week's British Medical Journal, said the positioning of the infant in the car seat caused a narrowing of the upper airways.

Prof. Alistair Gunn noted the children's heads were tilted forward with their jaws pressing down on their chests. Throat muscles relaxed when the infants were sleeping, exacerbating the problem.

Researchers also observed that half of the mothers in the study smoked, making the infants vulnerable to hypoxia or poor oxygenation.

Parents should be careful not to leave their children sleeping in car seats for extended periods of time, the study said.

"Providing a gap behind the head for the occiput [back of the skull] allows infants to avoid bending the head, with reduced frequency of episodes of desaturation," the authors said. "Modifying car safety seats so that head flexion is unlikely could avoid the risk of apparently life-threatening events."