A link between the prone sleeping position and crib death was first suspected about the same time, but the link did not become firm until more recent studies. Still, the findings have been controversial, in part because they come from studies in which the data was collected in interviews with bereaved parents after their babies had died. Under such conditions, parents might have been unintentionally biased in recalling the events surrounding a child's death.
Concern about such recall bias and a desire to sort out the factors by which the prone position raised the risk for crib death led researchers at the Menzies Center for Population Health Research to undertake several studies at the University of Tasmania. In one, they followed several thousand healthy infants from birth to try to identify the factors that distinguished those who died from crib death from the rest.
"We wanted to know: Are there some situations in which prone position is much worse than other situations?" Dr. Ponsonby said. Results in Tasmania
The Tasmanian team's latest study found a 20-fold increase in the risk for crib death for infants who usually slept prone on soft, fluffy mattresses filled with natural substances like ti-tree bark and kapok. Infants who slept prone on ordinary mattresses had triple the risk of infants who slept on their backs or sides.
Ti-tree bark is easily peeled from trees in Tasmania and the loose chips are packed inside a cotton cover to form a mattress. Similar soft mattresses are made from the long silky fibers of kapok, which covers the seeds of the ceiba tree.
Dr. Ponsonby said scientists now needed to learn whether infants suffocate because they rebreathe carbon dioxide while sleeping prone or whether ti-tree bark, kapok and other materials may harm infants directly or through allergic reactions.
The Tasmanian team also found that the risk of crib death was 12 times higher for infants who slept face down and who were swaddled in some kind of bedding. The authors suggested that the child's inability to change position might have been a factor. The risk was tripled for those who slept prone but not swaddled.
The risk was 10 times higher for infants who slept prone and who had nasal congestion, cough, chest noises, fever, vomiting or diarrhea on the day of death or the day before.
The risk increased 15 times when babies slept prone in heated rooms, which were defined as being more than 57 degrees Fahrenheit.
Dr. Ponsonby said her team had concluded that "healthy infants should not be placed prone to sleep" because "it is easier to change a baby's position than it is to make sure that all the other sleeping conditions are O.K."
An editorial in the same journal underscored the problem of scientific ignorance about crib death. In the editorial, Dr. Christian F. Poets of Children's Hospital in Hanover, Germany, and Dr. David P. Southall of the University of Keele in Stoke-on-Trent, England, wrote that the data "cannot explain why thousands of babies sleep face down on soft mattresses in overheated rooms and do not die, whereas some who sleep on their backs on hard mattresses in cool rooms die nevertheless."