Extreme Heat and Pregnancy Outcomes: A Scoping Review of the Epidemiological Evidence

Abstract

Background: Extreme heat caused by climate change is a major public health concern, disproportionately affecting poor and racialized communities. Gestational heat exposure is a well-established teratogen in animal studies, with a growing body of literature suggesting human pregnancies are similarly at risk. Characterization of extreme heat as a pregnancy risk is problematic due to nonstandard definitions of heat waves, and variable study designs. To better focus future research in this area, we conducted a scoping review to assess the effects of extreme heat on pregnancy outcomes. Methods: A scoping review of epidemiological studies investigating gestational heat-exposure and published 2010 and 2020, was conducted with an emphasis on study design, gestational windows of sensitivity, adverse pregnancy outcomes and characterization of environmental temperatures. Results: A sample of 84 studies was identified, predominantly set in high-income countries. Preterm birth, birthweight, congenital anomalies and stillbirth were the most common pregnancy outcome variables. Studies reported race/ethnicity and/or socioeconomic variables, however these were not always emphasized in the analysis. Conclusion: Use of precise temperature data by most studies avoided pitfalls of imprecise, regional definitions of heat waves, however inconsistent study design, and exposure windows are a significant challenge to systematic evaluation of this literature. Despite the high risk of extreme heat events and limited mitigation strategies in the global south, there is a significant gap in the epidemiological literature from these regions. Greater consistency in study design and exposure windows would enhance the rigor of this field.

The 2022 report of the Lancet Countdown on health and climate change: health at the mercy of fossil fuels

“…Climate change is leading to an increase in average global temperatures and in the frequency, intensity, and duration of heatwaves. 16 Exposure to extreme heat is associated with acute kidney injury, heatstroke, 17 adverse pregnancy outcomes, 18,19 worsened sleep patterns, 20 impacts on mental health, worsening of underlying cardiovascular and respiratory disease, and increases in non-accidental and injury-related deaths. 21 Exposure to extreme heat also affects health indirectly by restricting people's capacity to work and exercise.…”

Ecology and age, but not genetic ancestry, predict fetal loss in a wild baboon hybrid zone

“…The rate of fetal loss in Amboseli—approximately 1.4 out of 10 pregnancies (Beehner, Onderdonk, et al (2006) and this study)—is similar to some estimates of miscarriage rates for clinically recognized pregnancies in humans (e.g., ~10%–20% after implantation: Dimitriadis et al, 2020; Pinar et al, 2018). Moreover, abiotic environmental stressors, including high temperatures during pregnancy, have recently gained attention in potentially explaining adverse birth outcomes in humans, such as fetal loss (e.g., Hajdu & Hajdu, 2021; Kanner et al, 2020; Strand et al, 2012; Syed et al, 2022; but see Asamoah et al, 2018). For example, in low‐risk pregnant women in Utah, extreme heat exposure (>90th temperature percentile) increased the odds of stillbirth by ~5‐fold compared to exposure to moderate temperatures (Kanner et al, 2020).…”

Ecology and age, but not genetic ancestry, predict fetal loss in a wild baboon hybrid zone

“…The rate of fetal loss in Amboseli—approximately 1.4 out of ten pregnancies (Beehner, Onderdonk, et al, 2006 and this study)—is similar to some estimates of miscarriage rates for clinically recognized pregnancies in humans (e.g., ∼10-20% after implantation: Dimitriadis et al, 2020; Pinar et al, 2018). Moreover, abiotic environmental stressors, including high temperatures during pregnancy, have recently gained attention in potentially explaining adverse birth outcomes in humans, such as fetal loss (e.g., Hajdu & Hajdu, 2021; Kanner et al, 2020; Strand, Barnett, & Tong, 2012; Syed, O’Sullivan, & Phillips, 2022; but see Asamoah, Kjellstrom, & Ostergren, 2018). For example, in low-risk pregnant women in Utah, extreme heat exposure (>90 th temperature percentile) increased the odds of stillbirth by ∼5-fold compared to exposure to moderate temperatures (Kanner et al, 2020).…”