Abstract / Summary
The relationship between month or season of birth and long-term health outcomes remains an important but underexplored topic in public health. Evidence suggests that birth timing may influence disease susceptibility and broader health trajectories, reflecting interactions between biological and ecological factors. Clarifying these associations is essential for advancing prevention and a holistic view of health shaped by temporal exposures.
We conducted an umbrella review of systematic reviews and meta-analyses indexed in PubMed, Embase, and Web of Science up to 2 July 2025. Eligible studies reported quantitative estimates for associations between birth month/season and objectively measured health outcomes. We used random-effects models to produce pooled estimates and evaluated heterogeneity, small-study effects, and bias. We graded associations using established class-of-evidence criteria and assessed methodological quality with AMSTAR 2.
We included 10 meta-analyses, comprising 614 unique associations (defined as individual pairwise analyses between a specific birth month/season exposure and a distinct health outcome). Of these, 45 associations were statistically significant, and 25 were eligible for evidence grading. No association met class I criteria, but one (January birth and schizophrenia) reached class II. The most consistent findings involved neurodevelopmental and psychiatric outcomes, with winter births linked to a modest increase in schizophrenia risk (odds ratio (OR) = 1.05) and spring births to multiple sclerosis. Most autoimmune, oncological, and perinatal associations were graded as weak (class III-IV) due to small effect sizes or heterogeneity.
The strongest evidence for associations between month/season of birth and long-term health outcomes was restricted to specific neuropsychiatric outcomes, with most other associations graded as weak. Grading the credibility of these associations helps distinguish robust signals from exploratory findings, and provides a clear framework for future mechanistic and epidemiological investigations.
PROSPERO: CRD420251110595.