SIDS is given as the cause of death when doctors can find no cause for a a baby's unexpected death but do not suspect foul play.

The study suggests doctors could be able to determine whether such deaths require more investigation by measuring the amount of blood in the infant's lungs more carefully.

This way they can tell whether the death was caused by accidental or intentional smothering - which would require an inquest - or "genuine" SIDS, which would not.

Cause for concern

Publishing their findings in the Journal of Clinical Pathology, the researchers from Sheffield University said it was possible not all SIDs cases were correctly classified.

"We question whether all the cases until now regarded as SIDS really are 'true SIDS', particularly in view of our findings," they said.

"A convenient label is perhaps a disincentive for pathologists to look too closely into the circumstances of the death, especially as the histories available to them are often inadequate."

They used a digital examination technique to look for signs of excessive blood in the lungs of 62 infants who had died unexpectedly and suddenly.

The technique found evidence of bleeding into more than 5% of the total lung surface area in 73% of infants deemed to have suffered accidental smothering and in 45% suspected of suffering intentional smothering.

Less reliable

The more common technique - examining the lung tissue with a microscope - picked up less than two thirds of such cases.

The researchers said 11 of the 29 deaths originally recorded as SIDS would have been better classed as "unascertained" - which would have made an inquest necessary.

However, the term carries negative connotations and parents may feel accused if a pathologist comes to such a conclusion.

While the researchers admit that blood in the lungs does not necessarily indicate smothering, they point out that the two are linked strongly enough to cause concern.

Professor Peter Berry, of the department of paediatric pathology at St Michael's Hospital in Bristol, wrote a commentary on the findings in the journal.

He said the issue of accidental suffocation was controversial among parents in favour of sleeping with their babies. However, he added that secret video filming had established that parents did sometimes suffocate their babies on purpose.

Distressing terminology

However, pathologists should use the term "unascertained" carefully.

"'Unascertained' is not an accusation, but its indiscriminate use causes distress to parents," he said.

"It should not therefore be used lightly, nor to cover the pathologist's tail."

The Foundation for the Study of Infant Death agreed that the term should be used with care, and said that the definition of SIDS already is in part a death in which an autopsy has failed to discover a cause.

"FSID reiterates its call for every unexpected infant death to receive a thorough post mortem examination by a paediatric pathologist," a spokeswoman said.

She said the pathologist should have access to detailed family and case history, "to give the best chance of understanding why a baby has died".