The rapid and dramatic decline in SIDS mortality in the early 1990s has been attributed to the decrease in the proportion of infants sleeping prone.1
In New Zealand the original infant sleeping position recommendation was side or back, which was later changed to back or side and eventually to back only as more evidence became available that the side sleeping position was associated with an increased risk of SIDS compared to the back sleeping position. The prone sleeping position has been reported as being less than 4% in all surveys since 1991, and was 1.4% in this survey. The proportion of infants sleeping supine has increased steadily, with a corresponding decline in the side sleeping position. The side sleeping position doubles the risk of SIDS compared with the supine position.2,3
In New Zealand the SIDS prevention campaign was officially launched in February 1991, although the prevalence of the prone sleeping position had begun to decline from August 1989. Since then SIDS mortality has fallen a further 63%. A similar finding has been reported from England and Wales.5
The most likely explanation for this decline in mortality has been the decrease in infants sleeping on their side. Our study suggests that this change would result in a 39% reduction in SIDS, assuming that there is a causal relationship between sleeping position and SIDS. This is a minimum estimate as all infants that were placed on their side and back last night were classified as side sleepers. If these were considered back sleepers, the reduction in SIDS would be estimated to be 48%.
The strengths and limitations in this study must be considered. Women resident in the Auckland District Health Board region deliver only at National Women's Hospital apart from the small number who deliver at home (<4%). Thus the eligible sample is close to being representative of all births in the study region. The participation rate was 70% and although this is good for a postal survey, participants may be more likely to comply with health messages.
We conclude that there has been a further fall in SIDS following the initial considerable decline in SIDS following the recommendation to avoid placing infants prone to sleep, and this is likely to be due to the substantial increase in the proportion of infants placed to sleep on their back rather than on their side.