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

Acute heat exposure, which is increasing with climate change, likely increases preterm birth risk. However, few studies consider susceptible exposure windows for extreme heat events, particularly among historically unexposed populations. The 2021 Pacific Northwest Heat Dome produced the highest temperatures ever recorded in usually temperate Oregon and Washington State, offering an ideal study setting. We 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. Analysis included 716,096 exposed births across 67 monthly conception cohorts. We identified increased odds of preterm birth in cohorts exposed during months 2-3 (11% increase, 95% CI: [1%, 22%]) and 6-7 (14% increase, 95% CI: [5%, 24%]) of pregnancy. These findings partially agree with literature reporting elevated preterm birth risk after heat exposure in all trimesters. As extreme heat events are now expected once to twice per decade rather than once every thousand years, they pose risks to perinatal health.