Prenatal factors in the pathogenesis of schizophrenia.

The excess of winter-spring births among individuals suffering from schizophrenia provides strong evidence for the existence of some prenatally occurring factors in the pathogenesis of schizophrenia. Recent epidemiological findings suggest that maternal viral infections during the second trimester of pregnancy may play a crucial role in the aetiology of adult schizophrenia. A 'two-hit window' hypothesis of the mechanism of action of prenatal factors in the pathogenesis of schizophrenia suggests at least two time-specific prenatal aetiological events. The observed association between prenatal viral infection and increased incidence of adult schizophrenia need not be a direct cytotoxic result of the viral infection, but may be caused indirectly, for example from foetal minor cerebral haemorrhages produced by the anticoagulant effects of aspirin.