Prenatal and perinatal risk factors for psychiatric diseases of early onset

doi:10.1136/bmj.318.7198.1622

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Results are different if seasons are categorised differently

Pirkko Räsänen (pirkko.rasanen@oulu.fi), Senior research fellow,
Helinä Hakko, Biostatistician,
Marjo-Riitta Järvelin, Professor

Department of Psychiatry, University of Oulu, FIN-90210 Oulu, Finland
Department of Public Health Science and General Practice, FIN-90220 Oulu
Department of Neuroscience, Psychiatry, Ulleråker, University of Uppsala, S-750 17 Uppsala 17, Sweden

EDITOR—Hultman et al found that late winter births are associated with increased risks of schizophrenia and affective psychosis, the odds ratios being 1.4 and 1.5 respectively.1 The authors presented the excess of late winter births as one of their key messages, although the finding reached significance only among male subjects with affective psychosis.

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Odds ratios (with 95% confidence intervals) for incidence of schizophrenia and numbers of births in relation to season of birth in northern Finland 1966 birth cohort study

Using the northern Finland 1966 birth cohort (5636 male and 5381 female subjects), we partially replicated Hultman et al's study. The cohort is a genetically homogeneous population, and data are available on biological, socioeconomic, environmental, and health conditions for every individual from mid-pregnancy of their mother to the age of 31. Psychiatric diagnoses were made on the basis of extracts from the Finnish national hospital discharge register and a detailed diagnostic validation process.2 Birth seasonality was examined separately in those with schizophrenia (Diagnostic and Statistical Manual of Mental Disorders, third edition, revised, code 295; n=105) and those …