Objective.

—To determine the independent contribution of changes in infant sleep position to the recent decline in sudden infant death syndrome (SIDS) rate in Tasmania.

Design.

—(1) A comparison of the whole population incidence of SIDS before and after an intervention to reduce the prevalence of prone sleeping position. (2) A within-cohort analysis of the contribution of sleep position and other exposures to the decline in SIDS after the intervention.

Setting.

—Tasmania, Australia.

Participants.

—(1) All SIDS cases from 1975 through 1992. (2) A sample of one in five infants born in Tasmania who at perinatal assessment were scored to be at higher risk for SIDS since January 1988. Of 5534 infants included in the study, 39 later died of SIDS.

Interventions.

—Multiple public health activities to reduce the prevalence of the prone infant sleeping position in Tasmania and verbal information on the association between prone position and SIDS to cohort participants from May 1, 1991.

Main Outcome Measure.

—Sudden infant death syndrome incidence.

Results.

—The Tasmanian SIDS rate decreased (P<.01) from 3.8 (95% confidence interval [CI], 3.5 to 4.2) deaths per 1000 live births from 1975 through 1990 to a rate of 1.5(95% CI, 0.9 to 2.2) deaths per 1000 live births in 1991 through 1992. The SIDS mortality rate in the cohort by period of birth was 7.6 (95% CI, 4.9 to 10.3) deaths per 1000 live births for those born from May 1, 1988, through April 30, 1991, and 4.1 (95% CI, 1.3 7.0) deaths per 1000 infants for those born from May 1,1991, through October 31, 1992. The prevalence of usual prone sleeping position at 1 month of age was 29.9% and 4.3% in these two cohorts, respectively (adjusted odds ratio, 0.11; 95% CI, 0.08 to 0.13). Logistic regression demonstrated that 70% of the SIDS rate reduction in the cohort could be accounted for by the decreased prevalence of the prone sleeping position. Other factors examined individually contributed to less than 10% of the SIDS rate reduction.

Conclusions.

—The major contributing factor to the recent SIDS rate decline in Tasmania has been the reduction in the proportion of infants usually sleeping prone. EVIDENCE has

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