Seasonality of schizophrenia and stillbirths in The Netherlands

Studies from Denmark and the USA have reported a strong correlation between the seasonal pattern for stillbirths and the seasonal birth pattern for pe ople who develop schizophrenia. It has been suggested that the correlation could be caused by a common seasonal factor (e.g. intra-uterine infections during the third trimester of pregnancy), which produces death in some fetu ses and nonfatal brain changes in others, changes that are manifested in la ter life as schizophrenia. The aims of our study were (i) to assess the sea sonal patterns for stillbirths and for pre-schizophrenic births in The Neth erlands and (ii) to examine their relationship. The Dutch psychiatric registry provided data on all Dutch-born subjects who had been hospitalized at least once with a diagnosis of schizophrenia in t he period 1970-1994. We selected data on patients born in the period 1926-1 970 (n=29891). The government provided monthly numbers of live births and s tillbirths in the latter period. Seasonality of birth was examined using Po isson regression analysis. The risk of an admission for schizophrenia was highest for people born in t he months of May and June and lowest for those born in August and September . When the risk for subjects born in June was compared with the risk for su bjects born in September, the Relative Risk was 1.14 [95% confidence interv al (CI): 1.07 to 1.22]. The seasonal pattern of stillbirths was different, in that it showed a peak in the month of January. The low, however, as in s chizophrenia, occurred in the months of August and September. The two seaso nal patterns were found to be weakly correlated: Spearman's rank correlatio n coefficient rho=0.41 (95% CI: --0.22 to 0.80). This was the largest European study on birth seasonality in schizophrenia. The hypothesis that a common factor is responsible for a seasonal excess of stillbirths and for a seasonal birth excess of people who develop schizoph renia was not supported. The possibility remains, however, that a common fa ctor explains seasonal (birth rate) deficits in these disorders. (C) 2000 E lsevier Science B.V. All rights reserved.