Exposure to heat during pregnancy and preterm birth in North Carolina: disparities by residential greenness, urbanicity, and socioeconomic status

BACKGROUND AND AIM: Although previous literature suggested beneficial impacts of residential greenness on the association between heat exposure and several health outcomes, research is very limited for birth outcomes. We investigated the association between exposure to heat/heat waves during the last week of gestation and preterm birth (PTB) in North Carolina (NC) from 2003 to 2014. We evaluated disparities by residential greenness, urbanicity, and socioeconomic status (SES) in this association. METHODS: We obtained individual-level birth certificate data for NC from May to September for the years 2003-2014. We estimated daily mean temperature at each maternal residential address using Parameter-elevation Regressions on Independent Slopes Model (PRISM) data. We created 3 definitions of heat waves (≥95th, 97th, 99th percentile for warm-season temperature, for ≥2 consecutive days). The Normalized Difference Vegetation Index (NDVI) was used to assess residential greenness. We applied Cox proportional hazard models to estimate the association between heat/heat waves exposure and PTB after controlling for covariates. We evaluated whether the association between heat and risk of PTB varied by several individual and community characteristics. RESULTS:Of the 546,441 births, 8% were preterm births. Heat exposure during the last week before delivery was significantly associated with risk of PTB. A 1°C increase in temperature during the last week before delivery was associated with a hazard ratio of 1.01 (95% CI: 1.00, 1.02). Higher risk of heat for PTB was associated with some characteristics such as infants living in areas that were urbanized, low SES, or in the Coastal plains. We found significantly higher risk of PTB in areas with low greenness for urbanized area. For heat waves, we did not find significantly positive associations with PTB. CONCLUSIONS:Findings provide evidence that heat exposure during pregnancy increases the risk of PTB. These results have implications for future studies of disparity factors in the heat and birth outcomes associations. KEYWORDS: Heat, Heat waves, Greenness, Preterm birth, Urbanicity, Vulnerable population