Season of birth, an exogenous indicator of early life environment, has been related to higher risk of adverse psychiatric outcomes. According to literature, an excess of 5–8% of winter-spring births is found in individuals who later develop schizophrenia and bipolar disorder; this seasonal birth excess is also found in schizoaffective disorder (winter), major depression (March–May), and autism (March).

The objective of this study was to analyze the seasonal birth patterns of in-patients with psychiatric disorders.

Understand the relation between psychiatric disorders and season of birth during a 10year period in a Portuguese University Hospital.

Analyze the birth date distribution of 2202 in-patients between 2007 and 2016 and compare with the psychiatric diagnosis.

Patients’ diseases analyzed by birthday season: 60% of patients with schizoaffective disorder were born in winter-spring, 48.4% of mental retarded patients were born in autumn, 37% of dementia patients in winter, 77% of patients with delusional disorder in winter-spring, 78% of patients with Cluster A personality disorder in spring-summer and 71% of patients with substance abuse conditions in autumn-winter. No seasonal birth excess was found for bipolar affective disorder, schizophrenia, alcohol abuse, major depressive disorder or Cluster B personality disorder.

Our sample data shows evidence for a potential link between season of birth and risk for schizoaffective disorder, dementia, mental retardation, Cluster A personality disorder, delusional disorder and substance abuse. The attempt to explain seasonal birth patterns in psychiatric illnesses could serve to clarify the etiological bases of such disorders.