Heat Waves and Early Birth: Exploring Vulnerability by Individual- and Area-Level Factors

Area of Science:

• Environmental Health
• Reproductive Epidemiology
• Perinatal Health

Background:

• Extreme heat events are increasingly common and linked to adverse health outcomes.
• Previous research suggests a connection between heat exposure and adverse birth outcomes, including preterm birth.
• Understanding disparities in heat-related birth risks is crucial for public health interventions.

Purpose of the Study:

• To investigate the association between acute heat wave exposure and the risk of preterm birth (PTB) and early term birth (ETB).
• To examine how individual-level (maternal age, education) and area-level (social deprivation, land cover) factors modify these risks.
• To identify vulnerable subgroups experiencing heightened risks from heat exposure during pregnancy.

Main Methods:

• Utilized a time-stratified case-crossover design using vital records from multiple US states (1990-2017).
• Defined heat waves based on the 97.5th percentile of daily mean temperature within zip code tabulation areas (ZCTAs) during a four-day exposure window preceding birth.
• Stratified analyses by maternal and area-level characteristics, pooling results using inverse-variance weighting.

Main Results:

• Heat waves were associated with an increased risk of early term birth (ETB), particularly among younger mothers (<25 years), those with lower educational attainment (≤high school), and those in areas with high social deprivation and impervious land cover.
• Preterm birth (PTB) risks were also elevated in these subgroups, but also observed in more advantaged groups.
• The magnitude of increased odds for heat-related births ranged from 0% to 7.9% across all subgroups and outcomes.

Conclusions:

• Heat-related early term birth risk is disproportionately higher among socioeconomically disadvantaged populations.
• While preterm birth risks also increase with heat exposure, the patterns of vulnerability are less consistent across different socioeconomic strata.
• These findings highlight the need for targeted public health strategies to mitigate heat-related risks during pregnancy, especially for vulnerable groups.