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An increased risk of poor school performance for children born preterm has been shown in many studies, but whether this increase is attributable to preterm birth per se or to other factors associated with preterm birth has not been resolved.We used data from the Swedish Medical Birth Register, the Longitudinal Integration Database for Sickness Insurance and Labor Market Study, the Swedish Multigeneration Register, and the National School Register to link records comprising the Swedish birth cohorts from 1974 through 1991. Linear regression was used to assess the association between gestational duration and school performance, both with and without controlling for parental and socioeconomic factors. In a restricted analysis, we compared siblings only with each other.Preterm birth was strongly and negatively correlated with school performance. The distribution of school grades for children born at 31–33 weeks was on average 3.85 (95% confidence interval = −4.36 to −3.35) centiles lower than for children born at 40 weeks. For births at 22–24 weeks, the corresponding figure was −23.15 (−30.32 to −15.97). When taking confounders into account, the association remained. When restricting the analysis to siblings, however, the association between school performance and preterm birth after week 30 vanished completely, whereas it remained, less pronounced, for preterm birth before 30 weeks of gestation.Our study suggests that the association between school performance and preterm birth after 30 gestational weeks is attributable to factors other than preterm birth per se.