Preterm and Early-Term Birth, Heat Waves, and Our Changing Climate (2024)

Heat waves pose an escalating threat to human health in general and the health of pregnant people and infants in particular. Darrow et al¹ used 2 large datasets—a natality database that includes 55 million recent US births and a 1-km² grid of historical daily temperatures—to demonstrate an association between heat waves and rates of preterm and early-term births. The authors identify important positive associations of preterm and early-term birth with hotter, longer heat waves among lower socioeconomic status subgroups, as well as among Hispanic and non-Hispanic Black mothers. While associations of extreme heat with hospitalizations, suicides, mortality, and other health outcomes among older adults and the general population are increasingly well understood, the association of extreme heat with time to conception, pregnancy loss, and general health of pregnant individuals and infants are less-often discussed.^2,3 Furthermore, prenatal exposure to extreme heat may also have effects across the lifespan, including early childhood development, pubertal transition, and reproductive function. The findings of Darrow et al¹ are consistent with other studies that have demonstrated positive associations between extreme heat and increased rates of stillbirths, premature births, and lower-birth-weight babies. Importantly, Darrow et al¹ address the heterogeneity of most prior studies that have varying definitions of excessive heat. Their definition of hot days that adjusted for the 97.5th percentile of the mean temperature adds a critical understanding that pregnancies in areas that are typically cool and dry will be affected as much as those that are in areas that are typically hot and humid.

This study represents an important step forward in our understanding of the perinatal risk associated with heat wave exposure and builds on a rapidly expanding literature on the association of extreme heat with pregnancy and birth outcomes. In addition to the association of heat with fetal development, heat has especially powerful effects on infants, particularly those who are born premature or early term, and can lead to disruptions in sleep, mental health, and behavioral health. In a 2020 systematic review, Bekkar et al⁴ reviewed 68 studies comprising a total of more than 32 million births, finding that, of a subset of 10 studies investigating the impacts of heat, 9 identified associations between heat and adverse birth outcomes, such as preterm birth, low birth weight, and stillbirth. A review the following year identified 13 studies on links between heat exposure and congenital anomalies, principally with regard to heat exposure in the first trimester; while results included some null studies using self-recalled temperature, 10 of the studies found associations between heat exposure and congenital anomalies, including heart anomalies, neural tube defects, and facial and craniofacial abnormalities, as well as other conditions, including hypospadias.⁵ Although the association of exposure to extreme heat with adverse pregnancy and fetal outcomes is increasingly clear, the immediate and lifelong effects of prematurity are typically excluded from assessments of the effects of heat from both a health and economic perspective. As the authors note, prematurity is a leading cause of mortality in the neonatal period and increases a child’s risk of respiratory disease, cognitive impairments, and behavioral challenges. By failing to include these implications of preterm and early-term births in public health assessments, we vastly underestimate the effects of heat on population health. The sample size of more than 55 million births in the study by Darrow et al¹ compares favorably with previous studies, providing validation of the outcomes they studied throughout major US metropolitan areas.

These results are part of a larger trend toward the use of large nationally representative health outcomes datasets in combination with spatially and temporally resolved information on historical exposures, such as heat, air quality, and other hazards.⁶ These exposure datasets are being developed by both academic research groups and government agencies using combinations of historical weather station records, remote-sensing data, and interpolation and missing data smoothing techniques. Studies using such datasets represent a substantial step forward in terms of understanding individual environmental exposures across lengthy time spans and large geographic regions, and substantial opportunities exist to better understand the association between a variety of health outcomes and environmental hazards that are of increasing importance in the context of climate change.

Darrow et al¹ are timely in their engagement with heat, which is one of the principal physical threats that populations, particularly pregnant individuals and infants, who are especially sensitive to the effects of heat, will face because of global changes in the planet’s climate. In the US, the intensity of heat waves has increased by 24%, and the frequency of heat waves has more than doubled since the 1960s.⁷ These changes are largely attributable to the effects of anthropogenic climate change; future heat waves are expected to be increasingly hazardous as global, regional, and local temperatures increase.⁸

Mitigation of the intensity, frequency, and duration of heat waves, and ultimately the morbidity and mortality associated with future heat waves, depends on addressing the root causes of rising temperatures through rapid reductions in emissions of greenhouse gases around the world. Policies to limit warming on a global scale are expected to translate into less-dramatic increases in the health effects of future heat. Studies such as the one by Darrow et al,¹ which connect climate-responsive hazards with human effects that are easily understood and of wide public interest, such as preterm delivery, put a human face on the effects of climate change and can help policymakers and the public understand the scale of what is at risk and the importance of meaningful action.

Although solutions exist to address the root cause of warming and to adapt our systems and infrastructure to extreme heat, some amount of future warming and dangerous heat must be anticipated and planned for from the standpoint of national, regional, and metropolitan policy; public health surveillance and early warning; health care operations; and patient care. Understanding the scale of heat-related health harms and identifying which populations are at greatest risk through national studies that combine large datasets with high-resolution spatial and temporal exposure data provide an important path toward determining the hazards of greatest concern and the populations most in need of assistance or other forms of health protection, as well as tracking the effectiveness of policies and interventions to address these hazards and their effects.

Darrow et al¹ have provided a compelling set of results connecting heat waves with preterm and early-term delivery in the US. Their work has a variety of implications for different audiences. Physicians, nurses, and others working in direct patient care may choose to counsel pregnant patients and caregivers of infants on the risks associated with hot weather, drawing on this and other previous studies. In public health agencies, application of similar methods at local scales may provide opportunities to identify at-risk populations that can be prioritized to receive heat health protection interventions. For policymakers and members of the public, the increasingly clear connection between heat waves and harm to pregnant individuals and infants provides an important impetus to address root causes of our escalating exposure to heat waves and invest in adaptive strategies to reduce their effects at the scale of cities, neighborhoods, and individual homes. Opportunities abound for policymakers, businesses, and individuals to take action to address these distressing findings.

Published: May 24, 2024. doi:10.1001/jamanetworkopen.2024.12026

Open Access: This is an open access article distributed under the terms of the CC-BY License. © 2024 Dresser C et al. JAMA Network Open.

Corresponding Author: Kari C. Nadeau, MD, PhD, Department of Environmental Health, Harvard T. H. Chan School of Public Health, 665 Huntington Ave, Bldg 1, Boston, MA 02115 (knadeau@hsph.harvard.edu).

Conflict of Interest Disclosures: Dr Dresser reported receiving grants from Biogen, Americares, and Climate Central outside the submitted work. Dr Nadeau reported receiving grants from the National Institute of Allergy and Infectious Diseases. the National Heart, Lung, and Blood Institute, the National Institute of Environmental Health Sciences, and Food Allergy Research & Education; being a National Scientific Committee member of the Immune Tolerance Network and a National Scientific Committee member of National Institutes of Health clinical research centers during the conduct of the study; being a cofounder of, consultant for, and holding stock options in IgGenix; holding stock options in Seed Health, ClostraBio, and Cour; being a cofounder of and holding stock options in Alladapt; serving as a consultant for Excellergy, Red Tree Ventures, and Regeneron; and being a cofounder of Latitude outside the submitted work; in addition, Dr Nadeau reported having a patent for Mixed Allergen Composition and Methods for Using the Same issued, a patent for Granulocyte-Based Methods for Detecting and Monitoring Immune System Disorders issued, and a patent for Methods and Assays for Detecting and Quantifying Pure Subpopulations of White Blood Cells in Immune System Disorders issued. No other disclosures were reported.

Additional Contributions: We thank Lindsey Burkhardt, MD, MPH, Center on the Developing Child, Harvard University, for her input on this commentary; she was not compensated for this contribution.