SIDS outdoors and seasonality in Sweden 1975-1987

In order to link the reported increasing incidence of SIDS in Sweden to environmental or other risk factors, we compared birth records and circumstances of deaths in infants who died in 1975-1977 with those who died a decade later, 1985-1987. The number of liver births differed by only 6850 in these two periods. Data were acquired from autopsy records of the decreased infants and matched with their respective birth certificates which were obtained from the National Board of Health and Welfare. The reported postperinatal incidence of SIDS increased from 0.51 per 1000 in 1975-1977 to 0.93 per 1000 in 1985-1987. No significant differences in boy/girl ratio, birth weight, gestational age or age at death were found in the two periods. While the number of infants who died at night indoors in their own bed remained almost unchanged, the number of infants who died outdoors in baby carriages during the cold season increased almost four-fold. The observed increase in incidence could, to a large extent, be related to the increase in outdoor deaths. We speculate that increased incidence of smoking among young women as well as other changes in lifestyle may have been contributing factors.