Preconception blood pressure and risk of preterm birth: a large historical cohort study in a Chinese rural population

ObjectiveTo examine the relationship between preconception blood pressure (BP) and the risk of preterm birth (PTB) using a historical cohort of reproductive-aged women and girls from a Chinese rural population.DesignA historical cohort study established between 2010 and 2013 in People's Republic of China.SettingLocal family-planning service agencies and maternal–child care service centers.Patient(s)A total of 367,425 reproductive-aged women and girls who participated in the National Free Pre-pregnancy Checkups in 2010–2012 and subsequently had a live birth before October 2013.Intervention(s)None.Main Outcome Measure(s)Incidence rates of PTB in reproductive-aged women and girls across preconception BP levels.Result(s)Preterm births were defined as babies born alive at 28 to 37 weeks of pregnancy. Compared with the participants with a systolic BP of <120 mm Hg, the multivariable-adjusted odds ratio (OR) for PTB was 0.99 (95% confidence interval [CI]: 0.95–1.03]), 1.21 (95% CI: 1.11–1.32), 1.47 (95% CI: 1.22–1.77), 1.80 (95% CI: 1.25–2.59), and 1.81 (95% CI: 1.20–2.73) for the participants with systolic BP of 120–129, 130–139, 140–149, 150–159, and ≥160 mm Hg, respectively. When the participants with normal BP were used as the reference, the adjusted OR for PTB was 1.04 (95% CI: 1.01–1.08), 1.38 (95% CI: 1.25–1.53), and 1.54 (95% CI: 1.21–1.97) for the participants with prehypertension, stage-1 hypertension, and stage-2 hypertension, respectively. In addition, isolated systolic and diastolic hypertension increased the risk of PTB by 55% and 30%, respectively.Conclusion(s)Our results indicated a strong linear and independent relationship between BP levels and the risk of PTB in Chinese reproductive-aged women and girls. Maternal hypertension before pregnancy can significantly increase the risk of PTB.