Impacts of the Pacific Northwest Heat Dome on preterm birth rates in Oregon and Washington State

The 2021 Pacific Northwest Heat Dome produced the highest temperatures ever recorded in usually temperate Oregon and Washington State. Under climate change, events as extreme as the Heat Dome are forecasted once to twice per decade rather than once every thousand years. Among other health effects, extreme heat exposure increases risk of preterm birth. However, few studies have evaluated susceptible exposure windows for extreme heat during pregnancy, or quantified effects in historically unexposed populations like those in Oregon and Washington State.We treated the PNW Heat Dome as a natural experiment, and used 2016-2022 vital statistics records to estimate the gestation month-specific impact of the Heat Dome on preterm birth. Using an interrupted time series design with a synthetic control, we compared the observed odds of preterm birth in the exposed (in utero November 2020-July 2021) Oregon and Washington conception cohorts to counterfactual odds had the Heat Dome not happened. Our analysis included 716,096 exposed births across 67 monthly conception cohorts. Odds of preterm birth were higher than expected in cohorts exposed during months 2-3 (11% increase; 95% CI, 1-22) and months 6-7 (14% increase; 95% CI, 5-24) of pregnancy. We found that Heat Dome exposure during all trimesters increased preterm birth risk, and heat exposure in early pregnancy had a delayed effect on preterm birth risk, suggesting interventions to protect pregnant people from heat should span the entire pregnancy.