Exposure to wildfire smoke and heat stress can negatively affect birth outcomes for women, especially in climate-vulnerable neighborhoods, according to a new USC study. The study is one of the first to show that living in areas more susceptible to the harmful effects of climate-related exposures can significantly alter the effects of heat stress on adverse birth outcomes, even among women exposed to these conditions in the month before becoming pregnant. The research team examined 713 births among MADRES participants between 2016 and 2020. The team used data from CalFIRE (California Department of Forestry and Fire Protection) to identify the location, size, and duration of every wildland fire in southern California during the study period. They used the NOAA hazard mapping system to calculate the smoke density from each fire and applied sophisticated modeling methods to calculate ground-level smoke concentrations, estimating how much particle pollution—tiny droplets of black carbon, soot, and burned vegetation—the women in the cohort were exposed to during these events based on their daily residential location histories. They also pinpointed those LA neighborhoods that are most vulnerable to climate risks with mapping data from the California Urban Heat Island Index and the US Climate Vulnerability Index, two geospatial tools that analyze and map layers of data. They found that greater exposure to wildfire smoke and excessive heat during the month before conception and the first trimester of pregnancy was associated with greater odds of having a small-for-gestational-age (SGA) baby. For women living in the most climate-vulnerable neighborhoods, the study showed the effect of heat stress during preconception on the likelihood of an SGA birth almost doubled. The researchers also found an association between pregnant women exposed to moderate smoke-density days in the first trimester and having a low-birth-weight baby, or an infant weighing less than five pounds, eight ounces.