Sleeping position alone may not be a major factor in cot death, or sudden infant death syndrome.

The UK government has claimed massive success for its "Back to Sleep" campaign which urged parents to have their babies sleep on their backs, reversing earlier "expert" advice that they should be put to sleep on their fronts (or "prone").

But a study by American doctors suggests that the greatest risk of SIDS may occur when there is a combination of the baby sleeping on its front with other risk factors. These co-factors are claimed to be sleeping on mattresses filled with natural fibre; sleeping in heated rooms; swaddling; and recent illness.

These co-factors alone (without a prone sleeping position) did not increase risk; and similarly, nor did sleeping prone with none of these other variables, they claim.

A history of recent illness was significantly associated with SIDS among infants who slept prone but not among infants who slept in other positions. The risk of SIDS was greater among infants who slept prone on natural fiber mattresses than among infants who slept prone on other types of mattresses," they conclude.

The "natural fiber mattresses" were filled with flakes of ti tree bark or kapok fibre, and generally enclosed in a permeable cotton cover. The researchers say that the adverse effect of natural fibre mattresses is not negated by the use of sheepskins, of plastic or rubber mattress liners or of a quilt underneath the baby.

They conclude: "Healthy infants should not be placed prone to sleep. . If infants have to sleep in the prone position for a specific medical reason, they should be placed on a firm mattress and not swaddled." The researchers call for further studies to be conducted into why these factors seem to contribute to SIDS. New Eng J Med, 5 August 1993.