Deaths from cerebrovascular diseases correlated to month of birth: elevated risk of death from subarachnoid hemorrhage among summer-born

The authors investigated the population-based distribution of deaths from cerebrovascular diseases in Japan in 1986-1994 as a function of birth month, by examining death-certificate records. For a total of 853 981 people born in the years 1900-1959, the distribution of the number of deaths according to the month of birth was compared with the distribution expected from the monthly numbers of all births for each sex and for the corresponding birth decade. For those born between 1920 and 1949, there were significant discrepancies between the actual numbers of deaths from subarachnoid hemorrhage and the numbers expected, and these differences were related to the month of birth. Those born in summer, June-September, consistently had an elevated risk of death, particularly men, where the excess risk was 8%-23%. This tendency was also observed, less distinctly but significantly, for deaths from intracerebral hemorrhage, but was not observed for those dying from occlusion of the cerebral arteries. The observation that the risk of dying from subarachnoid hemorrhage was more than 10% higher among those born in the summer implies that at least one in ten deaths from subarachnoid hemorrhage has its origin at a perinatal stage. Although variations in hypertension in later life, which could possibly be "programmed" during the intra-uterine stages, could be an explanation for this observation, the disease-specific nature of the observation suggests the involvement of aneurysm formation, which is a predominant cause of subarachnoid hemorrhage