Child age influence on the seasonal death risk during the first year of life (Spain, 1975-1989).

Infant mortality rate reflects a population's health status, which depends on several factors, such as diseases, environment, child development, and other population characteristics that can show seasonal change throughout the year. This article deals with the relevance of child age on the probability of death in the different seasons throughout the first year of life, taking into account the illnesses causing death. The study was carried out using a file from the Spanish National Institute of Statistics. To eliminate bias related to health improvement, only the 49,761 urban deaths corresponding to capital of provinces, taking place between 1975 and 1989, were considered. The variables analyzed were: the month in which death took place, disease causing death, and the child age at death. Since many causes of death are very infrequent, the analysis was limited to a selection of the most common 45,155 death cases. The causes of death analyzed were: digestive, nervous and sensory, circulatory, and respiratory diseases, delivery difficulties, congenital anomalies, and signs and syndromes (WHO, ICD-9). The infant mortality rates are associated with the child age at death. Moreover, a general ontogenetic change of the death risk can be observed, with some particularities depending on the disease analyzed. In general, regarding all infant deaths, the obtained results suggest a higher risk of dying in winter for children surviving the first month of life, whereas a summer pattern, with a maximum mortality in June, July, and August, appeared when newborns younger than 1 week were considered.