Historical Shifts in Birth Month Distribution in Japan and the Impacts on Health Outcome Studies
Author(s)
Saki Takeshita, MD1, Yuichi Nishioka, MD, PhD1, Emiri Morita, MD1, Tomoya Myojin, MD, PhD2, Tatsuya Noda, MD, PhD1, Tomoaki Imamura, MD, PhD1.
1Nara Medical University, Kashihara, Japan, 2Hamamatsu University School of Medicine, Kashihara, Japan.
1Nara Medical University, Kashihara, Japan, 2Hamamatsu University School of Medicine, Kashihara, Japan.
OBJECTIVES: Although birth month has been reported to affect disease risk and longevity, the historical changes in the birth month distribution and its impacts on research have not been thoroughly studied. We examined the changes in the birth month distribution in Japan and their possible effects on health outcome research by investigating an association between birth month and mortality.
METHODS: The birth month distribution was analyzed using the national vital statistics of Japan from 1919 to 2023. We analysed the association between birth months and mortality by applying a regression discontinuity design using DeSC database.
RESULTS: 170,865,468 births were included from national vital statistics. The birth rate was consistently higher from January to March from 1919 for over 50 years, gradually diminishing over time. From the DeSC database, we included 10,245,469 people (4,602,783 men, 5,642,686 women) from 2018 to 2022. Mortality rates in the overall population, stratified by birth month and sex, were higher among individuals born in March than those born in April. In contrast, the analyses stratified by five-year birth year cohorts demonstrated no significant discontinuity in mortality at the April 1st cutoff, except for several cohorts.
CONCLUSIONS: The distribution of birth months has considerably changed in Japan. While crude analysis showed a significant discontinuity in mortality rate at April 1st cutoff, with a higher mortality rate in people born in March compared to those born in April, stratified analyses did not exhibit significant discontinuity, likely due to changes in the birth month trends. Given that the fiscal year in Japan begins in April, these results could be misinterpreted as the impact of the fiscal year system on mortality without stratified analysis. Stratified analysis by birth year should be conducted to account for differences in distribution, indicating that some previous studies may have reported spurious associations between birth months and health outcomes.
METHODS: The birth month distribution was analyzed using the national vital statistics of Japan from 1919 to 2023. We analysed the association between birth months and mortality by applying a regression discontinuity design using DeSC database.
RESULTS: 170,865,468 births were included from national vital statistics. The birth rate was consistently higher from January to March from 1919 for over 50 years, gradually diminishing over time. From the DeSC database, we included 10,245,469 people (4,602,783 men, 5,642,686 women) from 2018 to 2022. Mortality rates in the overall population, stratified by birth month and sex, were higher among individuals born in March than those born in April. In contrast, the analyses stratified by five-year birth year cohorts demonstrated no significant discontinuity in mortality at the April 1st cutoff, except for several cohorts.
CONCLUSIONS: The distribution of birth months has considerably changed in Japan. While crude analysis showed a significant discontinuity in mortality rate at April 1st cutoff, with a higher mortality rate in people born in March compared to those born in April, stratified analyses did not exhibit significant discontinuity, likely due to changes in the birth month trends. Given that the fiscal year in Japan begins in April, these results could be misinterpreted as the impact of the fiscal year system on mortality without stratified analysis. Stratified analysis by birth year should be conducted to account for differences in distribution, indicating that some previous studies may have reported spurious associations between birth months and health outcomes.
Conference/Value in Health Info
2025-11, ISPOR Europe 2025, Glasgow, Scotland
Value in Health, Volume 28, Issue S2
Code
MSR118
Topic
Epidemiology & Public Health, Methodological & Statistical Research, Study Approaches
Topic Subcategory
Confounding, Selection Bias Correction, Causal Inference
Disease
No Additional Disease & Conditions/Specialized Treatment Areas