Background and Hypothesis: Schizophrenia (SCZ) and bipolar disorder (BD) have shared genetic risk and clinical symptoms, yet the extent to which environmental risk factors are shared is not well known. We aimed to examine the associations of early-life environmental exposures with the risk of SCZ and BD. Study Design: We conducted a Swedish register-based nested case-control study using 4184 SCZ and 18 681 BD cases diagnosed 1988-2013, individually matched to 5 population-based controls by birth year, sex and birthplace. Conditional logistic regression was used to evaluate the risk of SCZ and BD by seasonality, severe prenatal infections, and perinatal factors. Study Results: Seasonality had similar patterns of risk for both disorders: Higher risk for births November-December; lower risk April-June. Experiencing any perinatal factor was associated with a significantly higher risk of SCZ (incidence rate ratio IRR 1.19, 95CI 1.11-1.63) and to a lesser extent BD (IRR 1.08, 95CI 1.05-1.12). Prenatal infections were only associated with a greater risk of SCZ (IRR 1.30, 95CI 1.04-1.63). In the mutually adjusted model, only perinatal factors were associated with outcomes. Several perinatal factors were associated with both disorders, but estimates were significantly higher for SCZ for low birth weight, low APGAR, and high parity. Congenital malformations were only associated with risk of SCZ, and jaundice with BD. Conclusions: Adverse perinatal factors and winter birth were the risk factors for both disorders, while severe prenatal infections were only risk a factor for SCZ. Early-life exposures were associated with a higher risk of both disorders, but may play a larger role in the development of SCZ than BD.
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