Restraints in infant car seats fit most newborns poorly, and low-birthweight infants in particular may not be well protected, Australian researchers say.

In their small study, fewer than one in five normal- and low-birthweight babies achieved a proper fit.

Historically, the smallest children's car seats were designed for babies weighing between 6.5 and 9 pounds (about 3 to 4 kg) and do not account for lower-weight infants, said lead author Julie Brown, a senior research scientist at Neuroscience Research Australia in Randwick.

The U.S. Centers for Disease Control and Prevention (CDC) classifies children born weighing less than 5.5 pounds (2.5 kg) as "low birthweight" and says 8 percent of U.S. babies fall into this category.

New car seat harnesses have become available for smaller infants, but there has been little research about how well they actually fit smaller babies, Brown told Reuters Health by email.

"Good harness fit is important for crash protection so we wanted to confirm that these new restraints would actually provide better harness fit," Brown said.

Her team conducted the study with 84 normal-weight and low-birthweight infants within a week of their discharge from a major hospital in Sydney.

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The researchers weighed each infant and the placed the baby in two different types of car seat: a rear-facing restraint generally recommended for younger infants and a convertible rear or front-facing restraint in its rear-facing mode. Some of the infants were also placed in specialized low-birthweight harnesses.

The research team adjusted the straps snugly enough so two adult fingers could fit between the strap and the baby. Then they assessed how well the harnesses fit the infants, including how well the shoulder and crotch straps fit the children and where the buckles sat on their bodies.

Only one in five normal-weight infants and one in eight low-birthweight infants had a good overall fit in the car seat restraints, the study team reports in Injury Prevention.

Among normal-weight infants, 84 percent had a good shoulder strap height, while only 67 percent of low-birthweight infants had a good score on this measure.

Additionally, only 51 percent of normal-weight babies and 42 percent of low-birthweight babies had a good fit of the crotch strap.

For both normal- and lower-weight infants, the rear-facing only restraints tended to have better scores for overall fit, shoulder strap height, buckle position and crotch strap gap, while the convertible restraints scored five times better on shoulder strap width.

All the low-birthweight infants achieved "good" scores for shoulder strap height and buckle position in the specialized low-birthweight harnesses and they were more likely to have good scores on crotch strap gap than in regular restraints.

Rebecca Ivers, a professor at the University of Sydney who studies infant car safety, noted that parents can help keep their children safe by knowing what the right seat is and how to use it.

"Appropriate use of child car seats significantly reduces the risk of death and serious injury in the event of a crash," said Ivers, who was not involved in the study.

"This means having straps tight enough, making sure no straps are twisted, that the seatbelt and buckles are fastened, and that any tether straps are correctly anchored," Ivers advised by email.

"All parents should remember that their children will get the best crash protection from age-appropriate restraints that are used correctly," Brown said. "Making sure the harness is always used and adjusted firmly is important for all children in all restraint systems."