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To examine whether blood pressure (BP) in the preconceptional period was associated with preterm birth in Chinese women.The data are from a large population-based cohort study established to evaluate the effectiveness of the campaign to prevent neural tube defects in 21 Chinese counties. We included 44 494 singleton live births delivered at gestational ages of 20–42 weeks to women who were registered before pregnancy in seven counties in southern China. Blood pressure was measured during registration by trained healthcare workers. We used logistic regression to evaluate the associations between prepregnancy blood pressure and increased risk of preterm birth, adjusting for potential confounders.The study size had 93% power (α=0.05) to detect an increase of 38% over the unexposed rate of 5.32% for preterm birth. The prevalence of hypertension of study population in prepregnancy was 4.55% (2023/44 494). The incidence of preterm birth was 5.73% for hypertension group and 5.32% for nonhypertension group. Compared with nonhypertension group, hypertension group did not show significant increased risk for preterm birth overall [adjusted risk ratio (RR)=1.10, 95% confidence interval (CI) 0.91–1.34], iatrogenic subtype [adjusted RR=1.21, 95% CI 0.78–1.88], or noniatrogenic subtype [adjusted RR=1.08, 95% CI 0.88–1.34]. When the participants with normal blood pressure were used as the reference, the adjusted RRs of noniatrogenic preterm birth were 0.79 (0.70–0.89) for prehypertensive women.Our results do not support the association between hypertension or higher blood pressure prior to pregnancy and the increased risk of preterm birth.