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

OBJECTIVE: To 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. DESIGN: A historical cohort study established between 2010 and 2013 in People's Republic of China. SETTING: Local 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.