Month of Birth and Mortality in Sweden: A Nation-Wide Population-Based Cohort Study

Abstract

BackgroundMonth of birth – an indicator for a variety of prenatal and early postnatal exposures – has been associated with life expectancy in adulthood. On the northern hemisphere, people born in the autumn live longer than those born during the spring. Only one study has followed a population longitudinally and no study has investigated the relation between month of birth and mortality risk below 50 years.Methods and resultsIn this nation-wide Swedish study, we included 6,194,745 subjects, using data from population-based health and administrative registries. The relation between month of birth (January – December) and mortality risk was assessed by fitting Cox proportional hazard regression models using attained age as the underlying time scale. Analyses were made for ages >30, >30 to 50, >50 to 80 and >80 years. Month of birth was a significant predictor of mortality in the age-spans >30, >50 to 80, and >80 years. In models adjusted for gender and education for ages >30 and >50 to 80 years, the lowest mortality was seen for people born in November and the highest mortality in those born in the spring/summer, peaking in May for mortality >30 years (25‰ excess hazard ratio compared to November, [95% confidence interval = 16–34 ]) and in April for mortality >50 to 80 years (42‰ excess hazard ratio compared to November, [95% confidence interval = 30–55]). In the ages >80 years the pattern was similar but the differences in mortality between birth months were smaller. For mortality within the age-span >30 to 50 years, results were inconclusive.ConclusionMonth of birth is associated to risk of mortality in ages above 50 years in Sweden. Further studies should aim at clarifying the mechanisms behind this association.

Month of Birth and Dementia Late in Life

“…The indicator was used in demographic studies by Doblhammer and Vaupel (2001), Doblhammer (2004), and Doblhammer et al (2005), who showed that life expectancy is still influenced by the month of birth among people who are currently elderly, but that this influence is becoming smaller over successive cohorts. Ueda et al (2013) confirmed these findings for Sweden in a study exploring the relationship between month of birth and late-life mortality with Swedish population register data. In general, research has shown that people born in the spring have a lower life expectancy than those born in the winter, and that in the southern hemisphere the pattern is shifted by half a year in line with the seasons there.…”

“…Meanwhile, the procreational habits hypothesis suggests that couples from different social strata have different preferences for giving birth in a particular season, and that couples might also differ in terms of their seasonal lifestyle. Doblhammer (2004) for Austria and Ueda et al (2013) for Sweden provided evidence that while the spring-born have a lower life expectancy, they also have a higher average educational level. Thus, controlling for socioeconomic status would serve to increase the month-of-birth differences.…”

Maternal age, birth order and other early-life factors: a family-level approach to exploring exceptional survival

“…This is relevant particularly in historical populations in which both nutritional status and the burden of infectious diseases exhibited great seasonal variation. Season of birth can also be an indicator of temperature, sun exposure and vitamin D levels (Flouris et al 2009;Ueda et al 2013). Second, in contrast with birth order or parental age at conception, this indicator is cyclical and largely exogenous; younger or older siblings may or may not be born during the same season and the distribution of births along the seasonal cycle captures variations that are largely unrelated to family background and other socio-economic or environmental conditions (Doblhammer 2004).…”

The Causal Link between Relative Age Effect and Entrepreneurship: Evidence from 17 Million Users across 49 Years on Taobao

“…That is, systematic differences among people born in different months come into existence well before the intervention of the education/training system. In particular, season of birth has been found to be associated with diverse physiological and psychological traits, such as birth weight (Currie andSchwandt 2013, Tanaka et al 2007), adult height (Tanaka et al 2007, Weber, Prossinger andSeidler 1998), life expectancy (Doblhammer and Vaupel 2001, Flouris et al 2009, Lowell and Davis 2010, Ueda et al 2013, and novelty/sensation seeking (Benjamin et al 1996, Ebstein et al 1996, Lesch et al 1996, among others.…”