Acute associations between heatwaves and preterm and early-term birth in 50 US metropolitan areas: a matched case-control study

Mengjiao Huang,Joshua L Warren,Matthew J Strickland,Heather A Holmes,Andrew J Newman,Megan Richards,Howard H Chang,Lyndsey A Darrow,Joshua V Garn,Yan Liu

doi:10.1186/s12940-021-00733-y

Abstract

BackgroundThe effect of heatwaves on adverse birth outcomes is not well understood and may vary by how heatwaves are defined. The study aims to examine acute associations between various heatwave definitions and preterm and early-term birth.MethodsUsing national vital records from 50 metropolitan statistical areas (MSAs) between 1982 and 1988, singleton preterm (< 37 weeks) and early-term births (37–38 weeks) were matched (1:1) to controls who completed at least 37 weeks or 39 weeks of gestation, respectively. Matching variables were MSA, maternal race, and maternal education. Sixty heatwave definitions including binary indicators for exposure to sustained heat, number of high heat days, and measures of heat intensity (the average degrees over the threshold in the past 7 days) based on the 97.5th percentile of MSA-specific temperature metrics, or the 85th percentile of positive excessive heat factor (EHF) were created. Odds ratios (OR) for heatwave exposures in the week preceding birth (or corresponding gestational week for controls) were estimated using conditional logistic regression adjusting for maternal age, marital status, and seasonality. Effect modification by maternal education, age, race/ethnicity, child sex, and region was assessed.ResultsThere were 615,329 preterm and 1,005,576 early-term case-control pairs in the analyses. For most definitions, exposure to heatwaves in the week before delivery was consistently associated with increased odds of early-term birth. Exposure to more high heat days and more degrees above the threshold yielded higher magnitude ORs. For exposure to 3 or more days over the 97.5th percentile of mean temperature in the past week compared to zero days, the OR was 1.027 for early-term birth (95%CI: 1.014, 1.039). Although we generally found null associations when assessing various heatwave definitions and preterm birth, ORs for both preterm and early-term birth were greater in magnitude among Hispanic and non-Hispanic black mothers.ConclusionAlthough associations varied across metrics and heatwave definitions, heatwaves were more consistently associated with early-term birth than with preterm birth. This study’s findings may have implications for prevention programs targeting vulnerable subgroups as climate change progresses.

Highlights

The effect of heatwaves on adverse birth outcomes is not well understood and may vary by how heatwaves are defined
The mean number of days per year which had 3 or more days over the threshold in the past week was between 8.5–9.9 days, whereas it ranged between 2.0–2.4 days for excessive heat factor (EHF)+ 85 (Table 1)
Our results suggest that heatwave duration and intensity are positively associated with an increase in early-term births

Summary

Introduction

The effect of heatwaves on adverse birth outcomes is not well understood and may vary by how heatwaves are defined. Preterm birth (birth less than 37 weeks of gestational age) is a leading cause of infant mortality and long-term neurological disabilities in children [1]. Early-term birth (37–38 weeks of gestational age) poses a measurable impact on an infant’s survival, growth, and development relative to those born at 39– 40 weeks [5, 6]. In the United States (US) during 1982– 1988, about 9.8% of live births were born preterm and 17.8% were born early-term [7]. In 2018, while there was a slight increase in preterm births (10%), early-term births increased significantly, affecting 26.5% of live births [8]. Despite this public health relevance, earlyterm birth has been understudied

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