Confinement of Winter Birth Excess in Schizophrenia to the Urban-born and its Gender Specificity

Abstract

BackgroundThe season of birth phenomenon in schizophrenia was reexamined in relation to place of birth, in order to test the hypothesis that a seasonal factor might operate preferentially among those who were urban-born.MethodThe seasonal distribution of births was examined among 3253 patients in two case registers having an ICD–9 diagnosis of schizophrenia and compared with the distribution of births among the normal population born in those catchment areas over the same period; those subjects born in population centres greater than 50 000 were defined as urban-born.ResultsPatients who were urban-born showed an excess of winter births relative to controls that was absent among their rural-born counterparts. On comparing patient groups, those who were urban-born were more likely to be born in the winter, while those who were rural-born were more likely to be born in the spring; this urban–rural distinction was confined essentially to female patients.ConclusionsThese findings might be accommodated most readily in terms of a spatially as well as seasonally varying environmental factor that is associated with urbanicity and to which female offspring are more vulnerable.