Birth month, birth season, and overall and cardiovascular disease mortality in US women: prospective cohort studyBMJ 2019; 367 doi: https://doi.org/10.1136/bmj.l6058 (Published 18 December 2019) Cite this as: BMJ 2019;367:l6058
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Studying birth month and season, cardiovascular and overall mortality: What about perinatal photoperiod?
Zhang and colleagues  observe birth month and season associations with cardiovascular disease (CVD) mortality. They suggest further research to confirm their observations and reveal possible mechanisms. In our view, rather than focusing on birth month and season alone, epidemiology may also want to target photoperiod (hours of daylight) in the perinatal time of life.
Photoperiod is the predominant time-cue for circadian clocks that co-govern myriad physiological processes (e.g. blood pressure and fate of nutrients – processes that can affect CVD). Importantly, perinatal photoperiods may imprint developing circadian systems and affect later life physiology. 
With perinatal time-of-year (i.e. season) as one co-determinant of perinatal photoperiod, researchers simply have to include perinatal location (i.e. latitude) as the other to develop dose metrics. Current study co-authors have previously assessed perinatal photoperiod associations with disease in the Nurses’ Health Study cohorts.  Within the current study population, they could now test how perinatal photoperiod associates with CVD and overall mortality.
Such studies would complement similar investigations underway with UK Biobank data.  If differential photoperiod-associated risks were observed, further research could target the hypothesized circadian imprinting mechanism. 
1. Zhang Y, Devore EE, Strohmaier S, et al. Birth month, birth season, and overall and cardiovascular disease mortality in US women: prospective cohort study. BMJ 2019;367:l6058.
2. Lewis P, Erren TC. Perinatal light imprinting of circadian clocks and systems (PLICCS): A signature of photoperiod around birth on circadian system stability and association with cancer. Chronobiology International 2017;34(6):782-801.
3. Devore EE, Chang SC, Okereke OI, et al. Photoperiod during maternal pregnancy and lifetime depression in offspring. Journal of Psychiatric Research 2018;104:169-75.
4. Perinatal Light Imprinting Circadian Clocks and Systems (PLICCS) and Associations with Disease in the UK BIOBANK. https://www.ukbiobank.ac.uk/2019/04/perinatal-light-imprinting-circadian....
Erren TC & Mohren J & Lewis P
Competing interests: No competing interests
More research is necessary to conclude if birth seasons truly influence cardiovascular disease risk in women in the United States.
I read with great interest Zhang et al’s article on birth season and cardiovascular disease mortality. 1 The authors are to be commended for doing a large study on birth month and future mortality risk and their Figure 1 is excellent in showing the seasonal trends of their results because the graph is circular instead of linear.
However, it is important to realize that traditional statistical inference methods, while commonly used, are inadequate to assess if a seasonal trend exists in a dataset. For example, a chi-square test when the data are grouped by months only examines if the counts are different by month and does not take the order of the months into account. Rogerson and others have proposed extensions to the Edwards test that are more appropriate for assessing if a seasonal trend is present. 2 Moreover, these concepts can be incorporated in multivariable regression modeling. 3, 4
Given these findings, women born in the spring and summer should not panic but instead everyone should focus on modifiable lifestyle factors to reduce their risk of cardiovascular disease. This study shows only slight differences in cardiovascular disease mortality by month and season with the confidence intervals of the hazard ratios largely overlapping over time. Future analyses (or a re-analysis of this dataset) using methods specifically designed to test for seasonality may further attenuate this perceived risk and provide more definitive conclusions on this thought-provoking topic.
1. Zhang Y, Devore EE, Strohmaier S, Grodstein F, Schernhammer ES. Birth month, birth season, and overall and cardiovascular disease mortality in US women: prospective cohort study.BMJ. 2019 Dec 18;367.
2. Rogerson PA. A generalization of Hewitt's test for seasonality. International Journal of Epidemiology. 1996 Jun 1;25(3):644-8.
3. Christiansen CF, Pedersen L, Sørensen HT, Rothman KJ. Methods to assess seasonal effects in epidemiological studies of infectious diseases—exemplified by application to the occurrence of meningococcal disease. Clinical Microbiology and Infection. 2012 Oct 1;18(10):963-9.
4. Christensen AL, Lundbye-Christensen S, Dethlefsen C. Poisson regression models outperform the geometrical model in estimating the peak-to-trough ratio of seasonal variation: a simulation study. Computer methods and programs in biomedicine. 2011 Dec 1;104(3):333-40.
Competing interests: No competing interests