Month of birth and cause-specific mortality between 50 and 80 years: a population-based longitudinal cohort study in Sweden.

Month of birth-a proxy for a variety of prenatal and early postnatal exposures including nutritional status, ambient temperature and infections-has been linked to mortality risk in adult life. We assessed the relation between month of birth and cause-specific mortality risk from cardiovascular diseases, infections, tumors and external causes-in ages of more than 50-80 years. In this nation-wide Swedish study, 4,240,338 subjects were followed from 1991 to 2010, using data from population-based health and administrative registries. The relation between month of birth and cause-specific mortality risk was assessed by fitting Cox proportional hazard regression models with attained age as the underlying time scale. In models adjusted for sex and education, month of birth was associated with cardiovascular and infectious mortality, but not with deaths from tumors or external causes. Compared with subjects born in November, a higher cardiovascular mortality was seen in subjects born from January through August, peaking in March/April [hazard ratio (HR) 1.066 compared to November, 95 % CI 1.045-1.086]. The mortality from infections was lowest for the birth months November and December and a distinct peak was observed for September-born (HR 1.108 compared to November, 95 % CI 1.046-1.175). Month of birth is associated with mortality from cardiovascular diseases and infections in ages of more than 50-80 years in Sweden. The mechanisms behind these associations remain to be elucidated.