The number of infant deaths caused by premature birth has been significantly underestimated, according to new research. More than a third of newborn deaths are caused by prematurity -- not 20 percent as was previously believed.

It is also the direct cause of 95 percent of the deaths that occur among infants delivered before the 32nd week of pregnancy.

The latest assessment is based on research findings reported by the Centers for Disease Control and Prevention in the journal Pediatrics.

The disparity is a result of listing premature birth as a cause of death only when no other reason is determined, even though the other reasons -- respiratory distress syndrome, for example -- are, in many cases, a consequence of prematurity.

An accurate picture of the true risk of premature birth will be a useful tool for physicians when advising women on the importance of prenatal care, the researchers maintained.

The research findings indicate that in the US, two-thirds of all deaths prior to the age of one year occur in babies who were born premature.

One in every eight infants born in 2004 -- the most recent year for which statistics are available -- was premature, according to the National Academy of Sciences. That represents a 30 percent increase over 1980.

Health officials have not been able to pinpoint the reason for the increase.