BioEd Online

The seeds of sudden infant death syndrome seem to be planted in part by conditions in the mother's womb. The finding shows it is not just genetic factors that determine whether babies are predisposed to the tragic condition. And although it will not change doctors' advice to parents, it may shed some light on the causes of these mysterious deaths.

Cot death, or sudden infant death syndrome (SIDS), is the name for the unexplained death of a baby, usually in its sleep. The condition strikes 1 in 2,000 babies in the developed world.

Most cases are isolated. But it is known that a woman who has had one baby die from SIDS is five times more likely than other women to have a second baby die. This was once thought to cast suspicion on a mother's ability to care for her children. But it is now accepted that there are biological reasons for this recurrence, although it hasn't been clear what these are.

Brothers and sisters

To investigate, a team led by Gordon Smith, an obstetrician at the University of Cambridge, UK, studied the medical records of more than a quarter of a million women in Scotland. They compared three groups: those who had given birth to two surviving children; those whose first baby had died from SIDS, but whose second child did not; and those who first child did not die, but whose second baby did.

They found that the babies of women whose first baby had died of SIDS were two to three times more likely to be smaller than usual and delivered early. What's more, women whose first baby was alive, but born small or early, were at increased risk of having a subsequent child die from SIDS. They publish their results in the Lancet¹.

Stunted growth and pre-term delivery are signs that the babies have experienced a poor environment in the womb. So the findings suggest that an inferior environment can make babies more vulnerable to SIDS, says Smith. "These women are tending to give birth to babies that are, on the whole, less healthy than the general population," he says.

The team then looked at why these women tended to have such complications. They were more likely to smoke, and to be unmarried, very young and living in areas of high deprivation.

Jigsaw puzzle

Although no one knows for sure precisely why the womb environment matters for cot death, it is possible that it affects the development of the part of the nervous system that controls breathing, says Smith. "But the long and the short of it is that these ideas are in the realm of speculation."

Richard Wilson, a trustee of the Foundation for the Study of Infant Deaths and a paediatrician at Kingston Hospital, London, says the work is an important contribution to piecing together the causes of SIDS. "This whole thing is a jigsaw and all the bits are being fitted together," he says.

The work will also help target information to the right people. All parents are told how to reduce cot death: doctors recommend that they do not smoke and put their baby to sleep on its back. But it is now especially important to focus on parents whose previous babies were born early or small, says Wilson.

In addition, the findings lend vital weight to previous work that shows the vast majority of cases of multiple cot deaths are not caused by child abuse. "It provides supportive evidence that most cot deaths are not murder, they are deaths from natural causes," says Wilson.