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AbstractObstetric complications increase the risk of schizophrenia. However, it is not known whether there is a causal relation or whether the association is mediated by genetic and/or shared environmental effects. The aim of this study was to investigate the associations between birthweight, other birth characteristics, and schizophrenia. Twin pairs discordant for schizophrenia will also control for unmeasured genetic and shared environmental effects. Prospectively filed obstetric records were used for a cohort analysis of 11,360 same-sexed twins, and within–twin pair analyses were conducted on 90 twin pairs discordant for schizophrenia. The results from the cohort study showed that low birthweight (less than or equal to 1999 grams; odds ratio [OR] 1.67, 95% confidence interval [CI] 0.88–3.14 and 2000–2299 grams; OR 1.79, 95% CI 1.07–3.01) and small head circumference (less than or equal to 31.5 cm; OR 1.61, 95% CI 1.03–2.51) were associated with later development of schizophrenia. The associations remained in the within-pair analyses. The association between low birthweight and schizophrenia is partly a function of reduced fetal growth. Fetal growth restriction seems to be associated with risk of schizophrenia independently of familial factors.

The Swedish Twin Registry (STR) today contains more than 194,000 twins and more than 75,000 pairs have zygosity determined by an intra-pair similarity algorithm, DNA, or by being of opposite sex. Of these, approximately 20,000, 25,000, and 30,000 pairs are monozygotic, same-sex dizygotic, and opposite-sex dizygotic pairs, respectively. Since its establishment in the late 1950s, the STR has been an important epidemiological resource for the study of genetic and environmental influences on a multitude of traits, behaviors, and diseases. Following large investments in the collection of biological specimens in the past 10 years we have now established a Swedish twin biobank with DNA from 45,000 twins and blood serum from 15,000 twins, which effectively has also transformed the registry into a powerful resource for molecular studies. We here describe the main projects within which the new collections of both biological samples as well as phenotypic measures have been collected. Coverage by year of birth, zygosity determination, ethnic heterogeneity, and influences of in vitro fertilization are also described.