Heat exposure in pregnancy: A gap between evidence and public health messaging

1.

The evidence has been mounting for decades: Extreme heat exposure increases pregnancy‐related risks for mothers and babies. In a changing climate, extreme heat exposure is increasing and will continue to increase worldwide. Low‐income countries are disproportionately affected due to high maternal and neonatal mortality and pronounced vulnerability to climate change effects. Despite the evidence on heat‐related risks during pregnancy, however, clear messages are not yet communicated to the global public in a way that can help them to recognize and reduce their risks. Strengthening the evidence base and improving risk communication remain urgent needs to help to prevent heat‐related mortality and morbidities.

2. HEAT EXPOSURE IN PREGNANCY

Heat exposure is associated with obstetric complications affecting both mother and offspring, including preterm birth, stillbirth, miscarriage, congenital anomalies, gestational diabetes, gestational hypertension and preeclampsia, cardiovascular events, placental abruption, and premature rupture of membranes [1, 2, 3]. Heat exposure is also associated with neonatal complications, including fetal distress, fetal growth restriction, and low birth weight [1]. The strength of evidence for these identified associations varies [2]. Published methodologies and study designs are heterogeneous, with differing definitions for extreme heat. While more work is needed to examine the mechanisms through which heat exposure affects pathophysiology in pregnancy, it appears that heat exposure can explain variations in maternal and perinatal mortality at the population level [4].

Three critical factors drive vulnerability during heatwaves. First, thermoregulation is the critical ability to maintain an internal core body temperature within a narrow, healthy range irrespective of surrounding environmental temperature. Thermoregulation is known to be difficult for people who are very young (infants) or very old (elderly) and for people who are pregnant. Second, heat exposure interacts with underlying conditions, including psychiatric and neurologic disorders, diabetes, cardiovascular disease, respiratory disease, and kidney disease [1, 5, 6]. In already compromised cardiovascular and respiratory systems, extreme heat may trigger acute health crises. Third and finally, socio‐economically marginalized people experience disproportionate exposure to heat and limited access to thermally safe spaces and health care. This includes outdoor workers and people experiencing limited mobility, social isolation, displacement, or unstable housing [7, 8]. Such risks will be greatest in populations that experience substantial wealth inequalities and/or complex humanitarian emergencies.

3. WHAT CAN BE DONE?

Proactive leaders will want to offer clinical and structural protection to pregnant women who are exposed to high temperatures. Unfortunately, a key barrier to action is the lack of evidence about which interventions effectively reduce maternal and fetal complications related to extreme heat. Many possible interventions have been suggested. Primary interventions would focus on reducing heat exposure and supporting bodily thermoregulation on hot days. Targeted counseling could be included in routine prenatal care, particularly for patients with increased risk from gestational diabetes or preeclampsia. Early warning systems could target pregnant women with tailored messaging to recognize risks and take protective actions [1]. The possibilities for household planning and adaptation depend on the wider environment, including a societal commitment to address the structural determinants that worsen vulnerability. Protective actions during periods of high heat could include access to cool, potable water, active cooling (air conditioning), and passive cooling such as shade or climate‐appropriate buildings designed and built to maximize comfort through high heat [1]. Existing programs that focus on maternal and newborn health could incorporate protection from heat effects. Strengthening the evidence base will require evaluating and comparing possible strategies and their underlying mechanisms. An implementation science perspective will help to identify feasible approaches and anticipate real‐world facilitators and barriers.

At present, interventions to reduce risks from air pollution are more commonly evaluated than interventions to reduce risks from extreme heat, with a greater focus on child outcomes than maternal outcomes [9]. Interventions that have been evaluated have often focused on individuals or families making household‐based changes, which do not address broader socio‐economic, cultural, and political realities [9]. Acknowledging the vulnerability of pregnant women may have adverse consequences, such as increased anxiety or reduced employment opportunities. This reality reinforces the importance of rigorous evidence that enables concrete policy recommendations and clear health messaging.

Further research is needed to understand the role of physiological adaptation to heat exposure. It is possible that populations experiencing ongoing high heat may be less affected by heat exposure than populations accustomed to cooler temperatures. An analysis of associations between temperature, precipitation, maternal mortality, and stillbirth identified statistically significant associations in the Democratic Republic of the Congo, where temperatures fluctuate, but not in Senegal, where temperatures are typically high and comparatively consistent [4, 10]. In the United States, a study on the association between extreme heat exposure and hospitalization in pregnancy determined that the identified effect was largely driven by women in historically cooler counties rather than those in historically warmer counties, suggesting that adaptation plays a role [11].

4. HEAT‐HEALTH ACTION PLANS

Heat‐health action plans offer an opportunity to translate knowledge into real‐world communications, as they systematically coordinate action to address and reduce the adverse health effects of extreme heat and heatwaves. Typically developed by public health and emergency management agencies working with stakeholders across sectors, heat‐health action plans can encompass early warning systems, health system preparations, coordination and governance, emergency response, public outreach and communications, and surveillance. Plans may focus on reducing heat exposures in particular settings, such as homes, workplaces, and health facilities. The success of heat‐health action plans often hinges on subnational and local inclusion and governance [12]. Heat‐health action plans naturally vary across contexts, focusing on locally relevant risks and locally feasible responses in a way that is culturally and linguistically resonant to their communities. Critically, they call attention to the populations that may be most at risk, proposing targeted outreach strategies. Data from high‐ and middle‐income countries dominate the existing literature on both heat‐health action plans and interventions that could protect pregnant women and their unborn children [9, 13]. More evidence is needed from low‐income countries.

Heat‐health action plans often highlight the vulnerability of elderly populations given their higher relative risk of heat‐related morbidity and mortality [14]. Their vulnerability reflects the intersection of age‐related declines in thermoregulation with the presence of chronic diseases and social isolation. Heat‐health action plans may also mention people with chronic diseases and those who work outdoors. Most commonly, however, they do not mention the particular vulnerability of pregnant women and their unborn offspring. One scoping review reported that only 52% of heat‐health action plans between 2004 and 2024 listed pregnant individuals as a key population to protect, and none comprehensively addressed extreme heat risks for mothers, neonates, and children [13]. A separate review of interventions targeted to vulnerable groups reported that pregnant women had the fewest number of targeted interventions, coming behind not only older people and people with chronic diseases, but also behind children and athletes [5]. Evaluations seeking to connect heat action plans to reductions in health impacts during extreme heat have not focused on pregnant populations [14]. The scarcity of attention paid to the vulnerability of pregnant women represents a missed opportunity for preventive care with generational implications, as adverse birth outcomes permanently alter the health trajectories of both mothers and newborns.

Prioritizing maternal health in our heat response will reduce maternal and perinatal morbidity and mortality. Given the growing literature that connects heat exposure to a diverse array of adverse birth outcomes, further implementation research must seek to identify which strategies are most effective in protecting pregnant women from extreme heat, particularly in low‐resource settings. Excluding pregnancy from heat‐health action plans represents a failure to protect women and newborns. As global temperatures continue to rise, our definition of heat vulnerability must evolve to reflect the biological reality of pregnancy, ensuring the protection of both mother and child.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

FUNDING INFORMATION

The authors received no specific funding for this work.

ETHICS STATEMENT

The authors have nothing to report.

DATA AVAILABILITY STATEMENT

Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.