Smoking and risk of cot death

“Nine out of ten cot death victims had mothers who smoked during pregnancy,” the_ Daily Mail_ and other newspapers reported.

The Independent on Sunday also reported that the study was thought to be “one of the most authoritative to date”, and that “women who smoke during pregnancy are four times more likely than non-smokers to see their child fall victim to cot death”.

The news coverage is based on a forthcoming article in a medical journal. The article is a review of the current evidence on the link between smoking and sudden infant death syndrome (SIDS). The article brings together what is already known about this association from different study designs.

There is a large and convincing body of evidence showing that smoking during pregnancy and around a baby increases the risk of cot death. The number of deaths has fallen over the last 20 years and this seems largely due to mothers positioning babies on their backs to sleep. However, maternal smoking levels have not decreased and smoking remains the largest modifiable risk factor for cot death. Public health campaigns should continue to promote the anti-smoking message. The NHS Smokefree service provides help for individuals who are trying to quit.

Where did the story come from?

Doctor Peter Fleming and Peter Blair from the Institute of Child Life and Health from the University of Bristol wrote this review. The review is not yet fully published, but the latest version is available on the internet as an “article in press” in the peer-reviewed medical journal Early Human Development .

What kind of scientific study was this?

This is a non-systematic review of research which has looked at the association between parental smoking and the risk of sudden infant death syndrome (SIDS). The authors report on 21 key studies, including their own previous studies, that have tried to assess the extent to which smoking increases the risk of cot death.

The authors also looked at other studies that have examined changes in risk factors over time and discuss the impact of smoking in the household after the baby is born. They show the change in sleeping position of infants before and after the “back to sleep” campaign, a public health programme aimed at changing maternal behaviour.

They finish off their discussion by considering the possible effects that smoking has on a developing foetus, such as impairing lung development, affecting the brain, and it’s impact on other important physiological processes.

What were the results of the study?

The review includes reports on studies including:

• A 1997 systematic review which found that found that mothers who smoked when they were pregnant increased their odds of SIDS by two or three times;
• A study conducted by the authors themselves in the UK in the 1990’s. This study found that maternal smoking while pregnant increased the odds of SIDS in the child 2.6 times (when other known risk factors such as low birth weight, parental drug and alcohol consumption etc. were taken into account).
• A review published in 2006 which found that babies in households where the father smoked but the mother did not, were 1.5 times more likely to die from SIDS than in households where father didn’t smoke.
• Another of the authors’ own studies (conducted in the 1990’s) which found that the risk of SIDS increased with the number of smokers in the household.

The authors also report on the results of a recent systematic review, which suggest that the risk associated with smoking has risen at the same time as mothers have successfully reduced the risk of cot death by positioning their infants on their backs to sleep.

What interpretations did the researchers draw from these results?

From their round up of key pieces of research, the authors say that the risk of unexpected infant death is greatly increased by both prenatal and postnatal exposure to tobacco smoke.

They believe that the recent prohibition on smoking in public places may lead to an increase in smoking at home and therefore an increase in childhood exposure to tobacco smoke.

The researchers, are quoted in the Independent as saying that “reduction of prenatal exposure to tobacco smoke (by reducing smoking in pregnancy) and of postnatal exposure to tobacco (e.g. by not allowing smoking in the home) will substantially reduce the risk of SIDS”.

What does the NHS Knowledge Service make of this study?

This review of the literature on a link between smoking and SIDS draws together recent studies and discusses them, re-emphasising the association between the two and smoking’s place as the number one modifiable risk factor for SIDS.

• The news coverage could give the impression that there is new evidence of a link between smoking and SIDS. However, smoking has been acknowledged as a risk factor for unexpected infant death for many years and public campaigns always carry anti-smoking messages.
• It is important to acknowledge that in the past 15 years, the number of SIDS deaths per 1,000 live births has fallen by 75%. A large observational study in the UK attributes this success to mothers who have taken the messages of the “back to sleep” campaign seriously. However, the same study found that the campaign has not reduced maternal smoking. In fact, the prevalence of maternal smoking in children dying from SIDS increased from 57% to 86% (about 9 out of 10) between 1984 and 2003.

The authors do well to again raise the issue of smoking at a time when the subject is pertinent because of the recent public smoking ban in the UK. Public health campaigns should consider ways to discourage smoking during pregnancy and in the home after the birth of a baby.

Sir Muir Gray adds...

This is another nail in the coffin of smoking - although pregnant women surely already have enough evidence; translating this evidence into practice is the top priority.