Exposure to wildfire smoke and extreme heat during the month before conception and the first trimester of pregnancy significantly increases the risk of delivering a small-for-gestational-age baby, particularly in climate-vulnerable neighborhoods, according to a study published in Environmental Science & Technology. Led by researchers at the Keck School of Medicine of the University of Southern California (USC), the investigation examines how environmental stressors and socioeconomic factors combine to threaten infant health.
How Wildfire Smoke and Heat Stress Impact Pregnancy
Pregnancy naturally compromises a woman’s ability to regulate body temperature, making heat stress a severe risk factor for both mother and fetus. According to lead author Roxana Khalili, a postdoctoral researcher in the Department of Population and Public Health Sciences at the Keck School of Medicine of USC, the timing of environmental exposures right around or before conception plays a critical role in birth outcomes. Poor air quality is closely associated with adverse health impacts, but researchers previously lacked extensive data on how wildfire smoke specifically affects pregnancies during these early windows.
To measure these hazards, the research team analyzed 713 births among participants in the MADRES cohort (Maternal and Developmental Risks from Environmental and Social Stressors), an ongoing study tracking pregnant women in Los Angeles between 2016 and 2020. The investigators utilized data from the California Department of Forestry and Fire Protection (CalFIRE) to track the location, size, and duration of wildland fires in Southern California. By combining National Oceanic and Atmospheric Administration (NOAA) hazard mapping systems with daily residential location histories, the team calculated ground-level smoke concentrations and particle pollution exposure, including black carbon and soot.
Assessing Neighborhood Vulnerability and Heat Exposure
To evaluate heat stress, the researchers gathered meteorological data covering daily temperatures, relative humidity, and wind speed. They also factored in measurements accounting for sun angle and cloud cover to assess direct sunlight exposure. To identify which areas face the highest climate risks, the team applied geospatial mapping tools from the California Urban Heat Island Index and the US Climate Vulnerability Index. These tools mapped the cumulative impacts of excessive heat, wildfire smoke, and local socioeconomic conditions.
The findings indicate that women living in neighborhoods with higher climate-related stressors and fewer resources experience exacerbated risks. Greater exposure to wildfire smoke and excessive heat during the preconception month and the first trimester correlated directly with higher odds of delivering a small-for-gestational-age (SGA) infant. An infant is classified as SGA when birth weight falls below the 10th percentile for gestational age, exposing the newborn to immediate complications like low oxygen levels alongside long-term metabolic, cognitive, and neurodevelopmental challenges.
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