Nine healthy babies had "apparently life-threatening" breathing problems while sleeping in car seats, the researchers say.
All of the babies survived and had no other breathing problems over the next year.
The researchers stress that infant car seats are "vital" for protecting babies in automobiles. But they add that redesigning the seats might reduce the risk of the breathing problems seen in these infants.
The nine cases all occurred in New Zealand between July 1999 and December 2000.
They were reported in BMJ (formerly the British Medical Journal) by Alistair Jan Gunn, MD, PhD, FRACP, and colleagues. Gunn is an associate professor in the physiology and pediatrics departments of New Zealand's University of Auckland.
All the babies were healthy. They were 5 weeks old, on average. One had been born prematurely.
Eight of the nine babies were sleeping in car seats placed on flat surfaces -- such as a floor in their home -- instead of in a car.
When placed on flat surfaces, the babies were in a "relatively upright position," the researchers say.
The infants' heads tipped forward, hampering breathing; they were too young to have good head control, Gunn's team explains.
Five of the babies' mothers were smokers. It's not clear if that had anything to do with the babies' breathing problems, say Gunn and colleagues.
Recent U.S. Study
In August 2006, Pediatrics reported an earlier study involving infant car safety seats.
The researchers in that study included T. Bernard Kinane, MD, of Massachusetts General Hospital for Children and Harvard Medical School.
Kinane and colleagues studied 67 healthy full-term newborns who were up to 1 week old.
Half the babies sat in a car seat for about an hour. The other half lay in a car "bed" for the same amount of time. A car bed allows a baby to lie down while restrained.
Monitors showed a drop in the babies' blood oxygen levels for "substantial" periods of time during the experiment, report Kinane and colleagues.
The scientists said "respiratory instability is a potential concern because of the upright position in the car seat" -- especially for premature newborns.
Problems could include a drop in the babies' oxygen levels, but it "generally has been accepted" that the crash protection offsets that risk, Kinane's team says.
"Unfortunately," they write, "the portability of car seats and busy contemporary lifestyles are resulting in infants spending extended periods of time in the car seat for reasons other than transport."
Overall, our study suggests that additional refinements of the car seat and bed are necessary to minimize or prevent respiratory compromise," the researchers write.
They called on manufacturers to address those issues.
"In the interim," Kinane's team writes, "we suggest caution when considering the use of these devices outside the setting of transportation in the car, for which they were designed."
The study was funded by Aprica, a Japan-based firm that makes car seats and other baby products.
Infant Car Seat Safety
Pediatric experts stress that car safety seats are essential for babies who ride in automobiles.
Still, babies in car seats need attention, according to the American Academy of Pediatrics (AAP).
"Don't leave your baby unattended in a car safety seat," states the AAP's web site.
"When your baby falls asleep in her car safety seat, it can be tempting to bring her inside and leave her alone in the seat, but this can be unsafe," the AAP says.
"Your baby can fall out of the seat, or the seat can fall over.
"And remember, placing the car safety seat on a shopping cart is unsafe too," the AAP continues.
"The best place for your baby to sleep is on her back in a safe crib," states the AAP.
It is also important to follow the manufacturer's instructions to use car seats safely.