Dose-response Relationship Between Maternal Blood Pressure in Pregnancy and Preterm Birth: Based on the Monitoring Data of 212,941 Pregnant Women in China

Background: Hypertensive disorders of pregnancy (HDP) is a generally accepted risk factor of preterm birth (PTB). Most related studies focus on the effects of various HDP on pregnancy outcomes. Based on large-scale maternal health monitoring data, this study analyzes the dose-response relationship between maternal Blood pressure (BP) in different trimesters and PTB. Methods: : Through the Maternal and Newborn Health Monitoring System in China, a total of 212,941 single-fetus pregnant women who delivered during 2014-2018 in 13 counties of 6 provinces in China were included in this study. BP level, distribution and changes in each trimester were described with linear trend test. Multivariate logistic regression analysis was performed to estimate the associations between BP groups in different gestational trimesters and PTB. Then a restricted cubic spline (RCS) was used to delineate the dose-response relationships between BP (both diastolic and systolic) during each trimester and PTB. Results: : The overall incidences of HDP and PTB were 7.07% and 4.04% respectively. The detection rates of HDP in the 1st, 2nd and 3rd trimesters were 1.03%, 2.06% and 6.23% respectively. Taking the group of normal BP as reference, the odds ratios(OR) of PTB for the groups of hypertension in the 1st, 2nd and 3rd trimesters was 3.23, 2.70 and 2.05 respectively (P<0.001). Hypotension in 3rd trimester was associated with a 1.5-fold higher risk of PTB (P<0.001). OR of PTB had a nonlinearly U-shaped association with SBP and DBP in the 1st, 2nd and 3rd trimesters. Conclusions: : The risks of PTB varied among pregnant women with the same BP in different trimesters. An increase of BP within the normal range during pregnancy could prevent PTB. Hypotension in 3rd trimester was associated with a high risk of PTB.