Seasonal differences in risk factors for sudden infant death syndrome

The aim of this study was to explore whether the risk of sudden infant deat h syndrome (SIDS) associated with prone sleeping position and other risk fa ctors varies with season. The study was a large nation-wide case-control st udy, which compared 485 cases with 1800 controls. Parents of 393 (81.0%) ca ses and 1591 (88.4%) controls were interviewed. Obstetric records were also examined. Infants dying in winter were older and had lower birthweights th an those dying in summer. The increased risk of SIDS associated with prone sleeping position was greater in winter than in summer. In contrast, the in creased risk of SIDS associated with excess thermal insulation and bed shar ing was less in winter than in summer. Prone sleeping position accounts for about half of the difference between the mortality rate in summer and that in winter. This suggests that some factor related to season modifies the e ffect orf prone sleeping position.