Parents should not let small babies sleep unattended in standard car safety seats, according to a study that suggests infants may run into breathing problems from dozing in this semi-hunched position.

Doctors at the University of Auckland in New Zealand examined the cases of 43 local babies who had turned “blue” from lack of oxygen in what was a clearly frightening and apparently life-threatening event. Nine of these infants had been restrained in a car safety seat that was suitable for their age, all but one of the seats was rear-facing and semi-reclining.

One of the nine was born prematurely, while the rest were full-term. All were found to be in normal health after their experience. When the experts reconstructed the scenes, using the infant’s own car seat, they found the baby’s head was invariably flexed forward. Its jaw pressed down on its chest, which caused a narrowing of the upper airway and led to breathing difficulties.

There were several factors that could have aggravated their difficulties. The infants were very young – average age five weeks – and were thus at a stage when head control is undeveloped. In addition, they were reported to be asleep at the time of the event, and the throat muscles relax through sleep.

Finally, half of the mothers in the study were smokers, and nicotine exposure could have encouraged hypoxia, or poor oxygenation.

The seats should be modified so that the babies head does not bend forward could avoid the risk.