The human health impacts of anthropogenic climate change continue to intensify. Perhaps most concerning is the rapid rise in ambient temperature, with 10 of the hottest years ever recorded having occurred over the last 15 years (IPCC, 2018; NASA, 2019). A robust literature has accumulated characterizing an extensive list of adverse health effects of heat exposure, identifying a number of groups particularly vulnerable (Ebi et al., 2018). The most recent group identified as highly vulnerable to heat exposure is expectant mothers (Bekkar et al., 2020; Chersich et al., 2020). A consistent relationship has been observed between increased ambient temperature and adverse pregnancy events, including increase in preterm delivery (PTD), small for gestational age, and stillbirth (Bekkar et al., 2020; Chersich et al., 2020) . Utilizing birth records and fetal death records from 2000-2004, we carried out two case-crossover studies assessing the impact of ambient temperature on preterm delivery and stillbirth across the contiguous United States, where 1 in 10 births results in a preterm delivery and 1 in 160 births results in a stillborn fetus. Our aim was to assess how increased temperature, singularly and in combination with air pollution exposure, impacts the odds of experiencing a preterm delivery or stillbirth (Aims 1 and 2). Our third aim, performed with a case-control study, expanded on the exploration of air pollution exposure, examining how traffic related air pollution (measured by maternal residential proximity to major roadways) impacts placental-associated stillbirth (Aim 3). We identified significantly increased odds of preterm delivery and stillbirth associated with a 10-degree Fahrenheit increase in average apparent temperature in the week preceding delivery for babies delivered in the warm season (May – October) and the meteorological summer (June – August). These increases were strongest in the Southern half of the United States and modified by maternal race/ethnicity for both preterm delivery and stillbirth. The increased odds were independent of air pollution exposure (ozone and PM2.5), which had no impact on the odds of preterm delivery or stillbirth. Furthermore, we did not observe a meaningful increase in overall odds of placental-associated stillbirth with increased proximity of the maternal residence to major roads. These studies contribute to the growing literature on the vulnerability of pregnant women to heat exposure and enhance the understanding of environmental risk factors of preterm birth and stillbirth, a chronically understudied health outcome. / 2023-05-14T00:00:00Z