July 23 (HealthDay News) -- Cases are very rare, but babies younger than one month old may be vulnerable to sudden unexplained death while seated, say researchers from the Montreal Children's Hospital.

A new study published July 19 in the online edition of Archives of Disease in Childhood analyzed all sudden unexpected deaths occurring among babies up to 12 months of age in Quebec, Canada, between 1991 and 2000.

During this time period, 534 babies died, but the cause of death was fully investigated in only 508 of the cases.

In 99 of the deaths, the cause of death was eventually determined, but for 409 babies, the cause of death remained unexplained.

Seventeen (3.3 percent) of the babies who died were seated, predominantly -- but not exclusively -- in car seats. Ten of these deaths were unexplained.

While premature babies were not at increased risk, babies younger than one month old were almost four times as likely to die suddenly while seated, compared to the older babies. And babies under one month old whose deaths were unexplained were more than seven times as likely to die while seated, compared to older babies with unexplained deaths.

The authors cautioned that the rate of deaths among seated babies is still small, and there's no question about the safety of using car seats for baby. Instead, it is important to take extra care when dealing with very young babies in any type of seat, the researchers said.