Globally, billions of people are exposed to elevated outdoor temperatures (i.e. heat) every year with many associated health implications. These numbers are expected to rise with global warming (i.e., anthropogenic climate change) in the United States and globally. Thus, heat is a worsening global health risk. Ambient heat is characterized in a number of ways including but not limited to maximum daily temperature, nighttime minimum temperature, and heat index (which combines temperature and humidity). Further, the definitions of extreme heat events or heat waves differ geographically, but are often defined as greater than 95th% of local seasonal temperatures for more than one day. Therefore, the association between heat and various health impacts can differ according to these different ways of defining heat exposure (Zhang et al [60]). The risk of heat-associated health impacts is also affected by the timing of exposure – such as a specific, critical window during pregnancy and whether the exposure is prolonged (chronic) or short-term (acute).

Documented heat-related health impacts include a wide range of outcomes from specific heat-related illnesses to cardiovascular disease, respiratory disease, mental health impacts, intimate-partner violence, mortality and also general increased demand of health care services. In a systematic review of health system demands of heat-related illness in Australia, researchers reported an increase in mortality, hospital visits, and ambulance demand across Australia for concerns relating to cardiovascular, respiratory, renal, and psychiatric issues (Mason et al [29]). In addition to health care encounters, there are also reported implications for compromised neurocognitive function and other subtle health effects with heat exposure [19]. Recent extreme heat events are raising public awareness of these risks and their entrenched inequities. For example, in June 2021, the Pacific Northwest of the United States exhibited a record-breaking heat wave that contributed to at least 1000 deaths and challenged power grid systems that sustain health care facilities [48].

The literature on populations vulnerable to extreme heat exposure have focused primarily on children and individuals over the age of 65, and increasingly on the role of structural racism in amplifying risk to Black, Indigenous and communities of color (BiPOC) communities. (Bikomeye et al [9], Mason et al [29,48]). While research gaps remain regarding specific vulnerabilities of pregnant individuals, significant evidence already suggests that heat increases various risks during pregnancy as well as the risk of negative health outcomes for the newborn. Additionally, the gaps in knowledge about the intersection of reproductive health and human-induced climate change has further exacerbated the gender disparities in climate research while poor populations of color experience layers of structural disadvantage. These issues require targeted research and multi-sector resources to address.

This article focuses on heat and its associations with adverse pregnancy and newborn health outcomes. Figure 1 underscores that the burden of heat may be underrepresented as the outcomes that are most easily studied, while more severe, are likely to impact a much smaller proportion of the at-risk population. Arguably, the majority of heat health impacts at the population level do not typically result in medical intervention but may exact a substantial proportion – or perhaps even the majority - of the heat-health burden (the proverbial base of the pyramid). In addition to describing the range of heat-related health outcomes, we summarize educational and clinical tool resources for clinicians and opportunities for near-term action using the precautionary principle framework. The Precautionary Principle acknowledges that limitations in scientific certainty should not preclude preventive measures to address threats to human health and ecosystems. The Wingspread Statement of 1998 articulated this principle in terms of the imperative of acting on what we know regarding environmental toxins (SEHN [50]). In the case of heat-related morbidity and mortality in pregnant and newborn populations, it is critical to equip health care providers with the tools to abide by this principle, as demonstrated in the field of environmental and prenatal toxicology (ACOG [3]).