Results are different if seasons are categorised differently

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.

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 …