Abstract
BACKGROUND AND AIM: The rate of severe maternal morbidity (SMM) has continued to increase in the United States. We aimed to examine if maternal exposure to extreme heat is associated with the risk of SMM. METHOD: We conducted a retrospective cohort study and included 425,722 singleton pregnancies from Kaiser Permanente Southern California in 2008-2018. Daily maximum temperature data at a 4 km resolution were obtained. An extreme heat day was defined as a day with the daily maximum temperature exceeding a certain threshold. Four percentiles (i.e., 75th, 90th, 95th, and 98th) of daily maximum temperature during the warm season (May-September) in 2007-2018 in Southern California were selected to define the extreme heat day. Long-term exposures included the proportions of extreme heat days during the entire pregnancy and by trimester. Short-term exposures were measured by 12 different heatwave exposures (combining the four thresholds with three durations, i.e., ≥ 2, ≥ 3, and ≥ 4 consecutive days) during the last gestational week. The discrete-time approach and Cox proportional hazards model were used to estimate the long- and short-term associations between heat exposure and SMM, respectively. Stratified analyses were conducted to examine vulnerable subpopulations and the effectiveness of mitigation measures. RESULTS: There were 7,767 (1.8%) SMM cases during delivery hospitalization in this study. Long-term associations with extreme heat were detected in the entire pregnancy and each trimester. Heatwave exposures during the last gestational week were associated with higher risks of SMM. Stronger associations were observed in mothers who were Hispanic, with lower education or incomes, or smokers. More green space may attenuate heat effects. CONCLUSIONS: Long- and short-term maternal exposure to extreme heat can be associated with an increased risk of SMM. More residential green space can be a potential mitigation strategy.