The excess of winter-spring births among individuals suffering from sc
hizophrenia provides strong evidence for the existence of some prenata
lly occurring factors in the pathogenesis of schizophrenia. Recent epi
demiological findings suggest that maternal viral infections during th
e second trimester of pregnancymay play a crucial role in the aetiolog
y of adult schizophrenia. A 'two-hit window' hypothesis of the mechani
sm 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 in
cidence of adult schizophrenia need not be a direct cytotoxic result o
f the viral infection, but may be caused indirectly, for example from
foetal minor cerebral haemorrhages produced by the anticoagulant effec
ts of aspirin.