Acute association between heatwaves and stillbirth in six US states

Abstract

Background Heatwaves are becoming more frequent and may acutely increase the risk of stillbirth, a rare and severe pregnancy outcome. Objectives Examine the association between multiple heatwave metrics and stillbirth in six U.S. states. Methods Data were collected from fetal death and birth records in California (1996–2017), Florida (1991–2017), Georgia (1994–2017), Kansas (1991–2017), New Jersey (1991–2015), and Oregon (1991–2017). Cases were matched to controls 1:4 based on maternal race/ethnicity, maternal education, and county, and exposure windows were aligned (gestational week prior to stillbirth). County-level temperature data were obtained from Daymet and linked to cases and controls by residential county and the exposure window. Five heatwave metrics (1 categorical, 3 dichotomous, 1 continuous) were created using different combinations of the duration and intensity of hot days (mean daily temperature exceeding the county-specific 97.5th percentile) during the exposure window, as well as a continuous measure of mean temperature during the exposure window modeled using natural splines to allow for nonlinear associations. State-specific odds ratios (ORs) and 95% confidence intervals (CI) were estimated using conditional logistic regression models. State-specific results were pooled using a fixed-effects meta-analysis. Results In our data set of 140,428 stillbirths (553,928 live birth controls), three of the five heatwave metrics examined were not associated with stillbirth. However, four consecutive hot days during the previous week was associated with a 3% increase in stillbirth risk (CI: 1.01, 1.06), and a 1 °C average increase over the threshold was associated with a 10% increase in stillbirth risk (CI: 1.04, 1.17). In continuous temperature analyses, there was a slight increased risk of stillbirth associated with extremely hot temperatures (≥ 35 °C). Discussion Most heat wave definitions examined were not associated with acute changes in stillbirth risk; however, the most extreme heatwave durations and temperatures were associated with a modest increase in stillbirth risk.

The Impact of Extreme Heat Exposure on Pregnant People and Neonates: A State of the Science Review

“…Asamoah et al suggested that each degree increase in maternal heat exposure (measured in WBGT) in warm regions was correlated with a 27% to 42% increase in likelihood of miscarriage or stillbirth (crude OR, 1.42; 95% CI, 1.00‐2.03) 37 . Other researchers in the United States found that incremental increases in temperature were associated with an increase in the risk and incidence of stillbirth 39,40 . Additionally, an average increase of 2.8°F (1°C) over the county 97.5th percentile temperature threshold was associated with a 10% increase in risk of stillbirth (OR, 1.1; CI, 1.04‐1.17) 40 …”

“…37 Other researchers in the United States found that incremental increases in temperature were associated with an increase in the risk and incidence of stillbirth. 39,40 Additionally, an average increase of 2.8°F (1°C) over the county 97.5th percentile temperature threshold was associated with a 10% increase in risk of stillbirth (OR, 1.1; CI, 1.04-1.17). 40 Some studies suggest that the timing of exposure to temperature extremes affects risk of stillbirth.…”

“…39,40 Additionally, an average increase of 2.8°F (1°C) over the county 97.5th percentile temperature threshold was associated with a 10% increase in risk of stillbirth (OR, 1.1; CI, 1.04-1.17). 40 Some studies suggest that the timing of exposure to temperature extremes affects risk of stillbirth. A cohort study in Brisbane, Australia found an increased risk of stillbirth with exposure to high ambient temperatures (measured in percentiles) during the second and third trimesters.…”

“…Negative heat-health outcomes for pregnant people included obstetric complications such as placental abruption, 20 gestational hypertension, 15,22 gestational diabetes mellitus (GDM), 23 cardiovascular events, 24 preterm birth (PTB), 5,11,13,17,[24][25][26][27][28][29][30][31][32][33][34] miscarriage, 22,35-37 stillbirth, 5,10,13,18,[25][26][27][37][38][39][40] and potential impacts on maternal mental health. 41 Neonatal complications included fetal distress, 42 neonatal intensive care unit (NICU) admissions, 43 fetal growth restriction or low birth weight, 4,13,25,26,29,44 congenital birth defects, 26,[45][46][47] diarrheal disease, [48][49][50] vectorborne diseases (VBDs),…”

“…Health care providers who care for pregnant people and infants should be aware of heat risks and recommend protective actions during high heat (See Table 1). 40,62,67 In their practice, perinatal care providers can optimize ventilation in clinics and labor suites, monitor hydration, and make cold, potable water available. 5 The WHO and WMO recommend that electric fans (such as ceiling fans) not be used when dry-bulb temperatures exceed 95°F (35°C), as circulating hot air can worsen heat stress.…”