Table 1.
Summary of previous studies on season of birth and dementia
| Study | Location | Cases | Reference population | Resulta | Conclusion |
|---|---|---|---|---|---|
| Philpot et al. [13] | United Kingdom | 534 AD cases | Birth rates from age-matched census sample | 75 versus 59.1 | Excess of first-quarter births among persons with AD with no family history of dementia (47 vs. 29.4). Effect not found in persons with family history (12 vs. 18.4) or combined |
| Dysken et al. [14] | United States of America | 727 Autopsy-confirmed AD cases | Birth rates from general population | 202 versus 184 | No differences between observed and expected quarterly birth rates, for both persons with and without family history |
| Vitiello et al. [15] | United States of America | 150 AD cases | 132 persons recruited from community | 41 versus 28 | No seasonal variation could be demonstrated. Did not change when excluding patients with family history of dementia |
| Henderson et al. [16] | Australia | 170 AD cases | Birth rates from geographically close community sample of similar age-group | 33 versus 38 | No evidence for seasonality of birth was found |
| Fratiglioni et al. [17] | Sweden | 121 AD cases | 345 age and sex matched individuals from same cohort study, without dementia | 1st quarter RR = 1.4 (95% CI 0.9–2.3)b | First quarter of the year as season of birth was not associated with an increased risk of AD, even when only looking at cases without family history |
| Vézina et al. [18] | Canada | 399 AD cases | (1) Population currently living in the area and (2) the population born during the same period in the area | May: 20 versus 33.8c | Significant deficit of births in the month of May |
| Ptok et al. [19] | Germany | 131 AD cases | 107 healthy elderly persons from same area | 20 versus 19.4 | Seasonal distribution of births was not found to increase the risk of AD. Controlling for APOE genotype did not change results |
| Doblhammer et al. [4] | Germany | 14,718 dementia cases | 149,225 persons (aged > 65) from the same health insurer | Winter: OR = 0.93d | Winter-born have the lowest risk of developing dementia. The highest risk was found among summer born. This difference was statistically significant when adjusting for age, sex and major vascular risk factors of dementia |
| Tolppanen et al. [20] | Finland | 70,719 AD cases | 282,862 age, sex and region-of-residence matched controls | Summer: OR = 1.03 (95% CI 1.00–1.05)e | No strong evidence that month or season of birth is related to risk of AD, although summer births (June–August) were associated with higher odds of AD compared to winter births. However, the absolute difference was only 0.5% (31.7% vs. 32.2%) |
| Ding et al. [21] | China | 1326 | 354,859 respondents of the Second China National Sample Survey on Disability, aged 60 years and older | OR = 0.76 (95% CI 0.65–0.90)f | Winter birth was associated with a lower prevalence of dementia, especially among those living in urban and northern areas of China |
AD Alzheimer’s disease, CI confidence interval, OR odds ratio, RR relative risk
aResults are shown for observed dementia births (cases with and without family history combined) versus expected dementia births (based on reference population) during the first quarter of the year, unless specified differently
bRisk ratio for AD comparing 1st quarter birth to birth during rest of the year, adjusted for age, sex and education
cAdjusted expected birth frequency based on population currently living in the area
dOdds ratio for dementia among winter born (Dec.–Feb.) compared to summer born (Jun.–Aug.), adjusted for age, sex and major vascular risk factors of dementia
eOdds ratio for dementia among summer born (Jun.–Aug.) compared to winter born (Nov.–Feb.)
fOdds ratio for dementia among summer born (Jun.–Aug.) compared to winter born (Dec.–Feb.), adjusted for age, gender, education, and annual household income