Failure to identify a male winter-born schizophrenia subgroup clinically

Systematic review and meta-analysis: Season of birth and schizophrenia risk

OBJECTIVE

Winter birth has been hypothesized to be associated with increased schizophrenia risk for nearly a century. Major hypotheses regarding the potential etiological risk factors for schizophrenia such as vitamin D deficiency and virus exposure in utero are predicated based on the observation that risk of schizophrenia is higher in children born in winter months.

METHODS

We conducted a systematic review and meta-analysis to examine the association between season and month of birth and risk of schizophrenia. We further investigated this relationship stratified by hemisphere.

RESULTS

Forty-three studies spanning 30 countries and territories and 440,039 individuals with schizophrenia were included in this meta-analysis. Winter births were associated with a small but statistically significant increased risk of schizophrenia (OR 1.05, 95 % CI 1.03-1.07, p < 0.0001) and summer births were associated with a small but statistically significant decreased risk of schizophrenia (OR 0.96, 95 % CI 0.94-0.98, p = 0.0001). Stratified subgroup analysis demonstrated no significant difference between hemispheres in the risk of schizophrenia for either winter or summer births.

CONCLUSIONS

Analysis using birth month data demonstrated a clear seasonal trend towards increased risk of schizophrenia being associated with winter birth months and decreased risk of schizophrenia in summer-to-fall months in the Northern but not Southern Hemisphere. These data suggest a small-but-substantial increased risk of schizophrenia in winter birth month. Further research needs to examine potential etiologic causes for this association.

Seasonality of births in schizophrenia and bipolar disorder: a review of the literature

More than 250 studies, covering 29 Northern and five Southern Hemisphere countries, have been published on the birth seasonality of individuals who develop schizophrenia and/or bipolar disorder. Despite methodological problems, the studies are remarkably consistent in showing a 5-8% winter-spring excess of births for both schizophrenia and mania/bipolar disorder. This seasonal birth excess is also found in schizoaffective disorder (December-March), major depression (March-May), and autism (March) but not in other psychiatric conditions with the possible exceptions of eating disorders and antisocial personality disorder. The seasonal birth pattern also may shift over time. Attempts to correlate the seasonal birth excess with specific features of schizophrenia suggest that winter-spring births are probably related to urban births and to a negative family history. Possible correlations include lesser severity of illness and neurophysiological measures. There appears to be no correlation with gender, social class, race, measurable pregnancy and birth complications, clinical subtypes, or neurological, neuropsychological, or neuroimaging measures. Virtually no correlation studies have been done for bipolar disorder. Regarding the cause of the birth seasonality, statistical artifact and parental procreational habits are unlikely explanations. Seasonal effects of genes, subtle pregnancy and birth complications, light and internal chemistry, toxins, nutrition, temperature/weather, and infectious agents or a combination of these are all viable possibilities.