The Mediation Effect of Maternal Blood Pressure on the Association between Heatwave Exposure and Preterm Birth in China

Background: Heatwave has been associated with higher risk of preterm birth (PTB) in numerous studies, but the potential mediators are unclear. Previous studies have suggested that maternal blood pressure (BP) may have a role in the associations between heatwave exposure and PTB.<br><br>Methods: We conducted a multi-center birth cohort encompassed nearly all singleton live births from 16 counties in 8 provinces across China during 2015-2018. In total, 197 080 singleton pregnancies who measured BP after 20 weeks’ gestation were included. Based on the BP variation throughout pregnancy,we defined mediator as maternal hypertension and a rise in BP during middle and late pregnancy. Nine types of heatwaves were defined with percentiles of year-round daily maximum temperature and durations. We examined the associations between heatwave, mediators and PTB using logistic models. Mediation analyses were conducted to to clarify the role of maternal hypertension and rise in BP of heatwave exposure and PTB.<br><br>Results: Depending on the all heatwave definition used, the odds ratios (ORs) and 95%CI of PTB associated with heatwave exposure during the entire 1-20 weeks ranged from 1.082 (1.025, 1.142) to 1.363 (1.292, 1.439). Heatwave exposure was also likely to increase the risk of maternal hypertension, and cause the rise in BP. The mediated proportion of different heatwaves on PTB by changing maternal hypertension could be up to 5%, while by rise in SBP or DBP could be as much as 13% and 11%. The mediated effect varied across different gestational periods, with stronger effects during 9-12 weeks. For the heatwave definitions, a higher mediation proportion was found in moderate heat compared with more extreme heat.<br><br>Conclusions: Our findings suggest that maternal heatwave exposure affect the PTB mediated by increasing a risk of maternal hypertension and causing rise in BP during the middle and late pregnancy.