Prenatal exposure to influenza does not cause schizophrenia

Claims have been made that maternal infection with influenza during pregnancy is a cause of schizophrenia in the child. These assertions are based upon some apparently significant associations between the timing of influenza epidemics in the general population and birth rates of people who later suffered from schizophrenia. Such associations have not been present in studies of the 1919 and 1957 epidemics, with sample sizes larger than those on which the claims were made. More decisively, in an investigation of the subsequent psychiatric admissions of people born a few months after the 1957 epidemic, it was found that the children of 945 mothers who actually suffered from influenza during the second trimester of pregnancy were at no greater risk of developing schizophrenia than children of mothers who were not infected. In contrast to the predictions of the influenza hypothesis of 26.5 extra cases by broad diagnostic criteria and 15.8 cases by narrow criteria, the numbers observed in children of mothers exposed to influenza in the second trimester were 3 and 1 cases respectively, close to the expected rate. It is concluded that prenatal influenza and schizophrenia are unrelated.