Seasonal longitudinal effects of winter birth on psychopathology, cognition, and functioning in schizophrenia-spectrum and affective disorders: Findings from the PsyCourse Study.

Aims Winter birth (WB) is a replicated risk factor for mental health conditions, potentially due to third-trimester Vitamin D deficiency and maternal viral infections. Beyond diagnosis, WB is associated with psychopathology, cognition, and functionality as epiphenomena. We analysed these outcomes in psychosis and affective disorders, considering illness duration and sex-specific effects.Methods We included 535 individuals with schizophrenia-spectrum and 667 with affective disorders from the PsyCourse Study, evaluated at four time points over 18 months. Participants were stratified by the birth season: winter vs. other seasons and by duration of illness (≥5 years). Psychopathology was assessed with the Positive and Negative Syndrome Scale (PANSS) for psychosis, Inventory of Depressive Symptomatology (IDS-C30) and Young Mania Rating Scale (YMRS) for affective disorders, functionality with the Global Assessment of Functioning Scale (GAF), and cognitive performance with Trail Making Tests A (TMT-A) and B (TMT-B), Verbal Digit Span, and Digit Symbol Test (DST). Linear mixed models adjusted for covariates were applied.Results No interaction effects between WB and diagnostic group or time remained significant after correction for multiple comparisons. In sex-stratified models, a significant WB × time interaction emerged for DST in females, with WB participants showing improvements over time in schizophrenia-spectrum disorders, and a crossover pattern in affective disorders.Conclusions WB has no robust effect on long-term outcomes on schizophrenia-spectrum or affective disorders. Subtle, sex-dependent effects on cognition were observed in females, with divergent longitudinal patterns between diagnostic groups, suggesting a possible early-life influence that attenuates over the course of illness.