Introduction

Climate change is the long-term change in the average weather patterns that define local, regional, and global climates. In 2007, the Intergovernmental Panel on Climate Change (IPCC) presented a large body of evidence supporting the presence of global warming and the impact of anthropogenic activities on the global climate [1]. The report suggests that a child born today will be living in an environment that is more than four degrees warmer than the average temperature during the preindustrial period and will experience significantly more frequent and intense environmental disasters such as heatwaves, wildfires, and hurricanes [2]. Since then, the number of published articles on health impacts of climate change increased by 182% [3]. These studies suggest that climate change is associated with many short- and long-term health effects across the human lifespan, ranging from dehydration to heatstroke, respiratory diseases, infectious diseases, mental health complications, cardiovascular disease, and even death [2,3,4]. As such, climate change is recognized as the “biggest global health threat of the twenty-first century.” [5].

Pregnant women and the growing fetus experience an extraordinary time with many tightly regulated physiologic and psychologic changes [6, 7]. Any environmental perturbation during this sensitive period could have both immediate and life-long consequences for both mother and offspring [8, 9]. However, research on the health impacts of climate change on pregnancy outcomes is highly limited [10], contributing to the lack of consistent guidelines on how to adapt to and/or mitigate climate impacts among pregnant women. In fact, pregnant women have only been recently added as a vulnerable group with respect to environmental exposures such as air pollution and extreme heat [11, 12]. The objective of this narrative review is to summarize recent literature regarding how climate change and related environmental disasters influence pregnancy health.

Climate impacts on pregnancy health can be conceptualized to involve (a) direct impacts via discrete environmental disasters, (b) indirect impacts through changes in the natural environment, and (c) indirect impacts through changes in the social environment (Fig. 1). It is important to note that although direct and indirect impacts are commonly evaluated separately, they often occur simultaneously and have a synergistic and/or cascading impact on pregnancy health.

Fig. 1
figure 1

Impacts of climate change on pregnancy outcomes

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Direct Impacts of Climate Change on Pregnancy Outcomes

Climate change is expected to cause more frequent and intense climate-related environmental disasters such as heatwaves; wildfires; and extreme weather events such as drought, hurricane, and flood [1, 13]. As described below, accumulating evidence suggests positive associations between prenatal exposures to these events and adverse pregnancy outcomes. More importantly, populations who have the least access to the world’s resources and contribute least to climate change bear a disproportionately higher burden. [5]

Heat Events

The average annual number of heatwaves, defined as a series of unusually hot days, in the USA increased from two in the 1960s to six in the 2010s. [14] The length and average temperature of individual heatwaves have also increased significantly in recent decades [14]. Pregnant women are more prone to heat stress than non-pregnant women due to their compromised thermoregulation and homeostasis ability. A recent meta-analysis of 70 studies across 27 countries examines the impact of high temperature on preterm birth, low birthweight, and stillbirth [15]. Summary estimates suggest a 16% higher risk of preterm birth during heatwave days compared to on non-heatwave days [15]. Furthermore, each additional degree Fahrenheit (0.56 °C) was associated with a 5% increased risk. Potential susceptible exposure windows for preterm birth include 1 month preconception, the month of conception, first trimester, second trimester, and last week of pregnancy, suggesting both acute and chronic exposures are relevant [15]. The meta-analysis also showed that the low birthweight rate was 9% higher during periods with hotter than usual temperature, with babies on average 26 g lighter [15]. Additionally, stillbirth risk was 46% higher during heatwave compared to non-heatwave days, with risk increment of 5% for each additional degree Fahrenheit. Early pregnancy appears to be the most susceptible window of exposure for stillbirth [15, 16]. Meanwhile, high temperatures have also been linked to other serious pregnancy outcomes such as premature rupture of membranes [17], gestational cardiovascular events [18], gestational hypertension and preeclampsia [20, 33,34,35,36]. Heat exposure can also stimulate maternal antidiuretic hormone and oxytocin, both of which reduce uterine blood flow, and switch fetal metabolism from anabolic to catabolic pathways [37]. Extremely low or high temperatures are also associated with emotional stress during pregnancy, which may further exacerbate endocrine, endothelial, and placental dysfunctions. [38].

Wildfires

The number of wildfires and area burned increased significantly in recent years and is expected to continue [14]. A recent meta-analysis reported that, as of June 2020, eight epidemiologic studies in four countries (covering a total of ~ 1.7 million births) have evaluated the effects of wildfire exposures on adverse pregnancy outcomes [39]. The analysis shows birthweight is most consistently impacted by wildfires. More specifically, six of the seven studies that evaluated associations between wildfire exposures and birthweight reported a significant link with birthweight reduction [39]. Two studies (out of four) identified associations between wildfire exposures with preterm birth risk [40, 41], and one study with fetal loss and infant mortality [42]. A more recent California study suggests that each additional day of exposure to wildfire smoke, as estimated by a satellite-based approach, was associated with ~ 0.5% increase in the risk of preterm birth [43]. The authors also estimated that wildfires may contribute to almost 4% of observed preterm births. Few studies evaluated critical windows of exposure, although those that did suggest that the second and third trimester may be more sensitive, and those from poorer neighborhoods and smokers may be more vulnerable [27, 39, 40, 43, 44]. Of note, there is significant heterogeneity between studies, especially in how exposure is defined, which include distance to wildfires, fine particle concentration, heat spots from satellite images, and aerosol index. Some studies compared exposures between areas with varying degree of exposures, while others compared exposures between time periods with and without wildfires.

Although research on the impacts of wildfires on pregnancy health is still in its infancy, much more mature literature on the effects of combustion products, smoking, and air pollution on pregnancy health can potentially inform what we can expect from wildfires. Wildfire smoke is a complex mixture containing gaseous pollutants, organic compounds, and fine particles, much of which is similar to pollution from combustion, smoking, and other sources [45, 46]. These pollutants may impact pregnancy health through a few mechanisms including endothelial dysfunction, endocrine disruption, immunologic dysfunction, systemic inflammation, and oxidative stress [47,48,49,144]. Water has implications not only for drinking but also for sanitation, a major contributor to the global burden of disease [145, 146]. Although the number of studies on water availability and pregnancy outcomes is still limited, existing evidence suggests that limited water access can cause dehydration among pregnant women, leading to increased risk low birthweight and preterm birth [147,148,149,150]. Lack of safe water supply can also increase the likelihood of consumption of contaminated water resulting in poor pregnancy health outcomes. [151,152,153].

Shifts in Vector and Pathogen Distribution

The World Health Organization reports that in 2017, 435 million of the global population rely on water from unprotected sources such as wells and springs, and 144 million depend on untreated surfaces such as lakes, ponds, rivers, and streams [154]. Water shortages, coupled with warming temperature and extreme weather events, lead to even more opportunities for widespread distribution of and exposures to common water-borne pathogens such as cyanobacteria, enteric bacteria, parasites, and Vibrio bacteria [155]. Pregnant women can be exposed to these pathogens through drinking, recreational use, and/or ingestion of shellfish [156]. Waterborne infections are known to cause many pregnancy and fetal complications including maternal biliary ascariasis, septicemia in pregnancy, spontaneous miscarriage, preterm delivery, intrauterine growth restriction, and birth defects. [157].

The distribution and activity of some important vectors such as ticks and mosquitoes have expanded northward and to higher elevations in recent years. Accordingly, the incidence of malaria is expected to increase in previously unaffected areas, which will be further exacerbated by increased population density and forced migration caused by extreme weather events and food/water shortage [158]. Malaria infection has been known to cause severe malaria-induced anemia during pregnancy and increase the risk of intrauterine growth restriction, preterm birth, and low birthweight [159, 160]. Meanwhile, dengue transmission is also expected to increase, with approximately 5–6 billion people considered at risk by the end of the century [143]. Maternal dengue infection is capable of vertical transmission to the fetus, causing fetal or perinatal mortality. This infection is also known to increase the risk of maternal mortality, pre-eclampsia, eclampsia, preterm birth, low birthweight, and C-section  [163].

Even in areas not impacted by forced migration, rising temperatures in cities densely populated areas can form urban heat islands, which are areas that are significantly warmer than their surroundings as a result of land use and waste heat generated by energy use. Many US cities have temperatures up to 7°F (~ 4°Celsius) warmer than their surroundings [169]. Although the impact of heat island on pregnancy health has not received attention, studies have shown that minoritized populations are more likely to be exposed, which may further contribute to reproductive health disparities. [170, 171].

Implications

An increasing body of evidence suggests that climate change adversely impacts pregnancy health through both direct and indirect pathways. The Developmental Origin of Health and Disease theory [8] suggests environmental perturbation(s) during pregnancy (and other developmentally critical periods) have significant immediate and long-term health impact for both mother and offspring (Fig. 2). Pregnant women who have pregnancy complications are more likely to experience recurrence in subsequent pregnancy [172] and have greater risk of cardiovascular and metabolic diseases later in life [173, 174]. Babies affected by preterm birth and low birthweight are more likely to develop subsequent health complications including neurodevelopmental disorders, immunologic complications, obesity, and cardiovascular diseases, all of which put them at higher risk of adverse pregnancy outcomes if they become pregnant [175]. Accordingly, the impact of climate change on pregnancy health is not limited to this time window, but may propagate health risk across an individual’s lifespan and even into future generations (Fig. 2). Although beyond the scope of this review, the health impacts of the changing climate on paternal health can also further contribute to adverse fetal outcomes. [176].

Fig. 2
figure 2

The cycle of health impacts following climate related exposures during pregnancy

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Despite challenges, concerted efforts for adaptation and mitigation should be continued and strengthened. First, educational efforts and resources to raise awareness will facilitate behavioral changes and encourage public support for actions needed to reduce emissions and mitigate health impacts. Although climate change is generally known to the public, populations most impacted by its consequences have limited knowledge, power, and resources to mitigate its impact [177]. Thus, it is pertinent that any adaptations and mitigation efforts target these underserved populations. Such efforts should also target healthcare professionals, who, through patient-provider relationship, can support vulnerability reduction strategies. A US survey of health professional suggests that nearly 0% of OBGYN practitioners discuss environmental impacts of health with their patients [178]. Data also show that the majority of healthcare providers recognize the presence of climate change as a major threat to human health [179] but the lack of time, training, resources, and guidance are major barriers. Thus, provision of training, patient educational materials, and clear policy guidance will empower healthcare providers to become a critical part in health mitigation efforts.

Meanwhile, local, regional, and international policies aiming to reduce emission should be continued and strengthened. Even with massive international agreements such as the Kyoto Protocol and Paris Agreement, some experts are concerned that they are not aggressive enough. Solutions could include strengthening the commitment and implementation of strategies for accountability. Local policies have also proven to be extremely important. When the Trump administration withdrew from the Paris Agreement, more than 600 local governments still had detailed climate action plans [180]. Furthermore, efforts to reduce waste and increase renewable energy consumption, energy efficient appliances, and carbon-neutrality should be reinforced.

Lastly, more research is needed as we are only at the beginning of understanding how climate change and its environmental consequences impact pregnancy health. Research efforts are especially needed in areas with the most impact, which are often underserved with few resources. Research priorities should include the following:

  1. 1.

    Development of novel methods and models to better quantify climate change and its impacts on health, while considering confounding and the complex interaction between extreme climatic events, and between individual and environmental factors

  2. 2.

    Identification of susceptible window(s) of exposure and sensitive subgroups for specific climatic events

  3. 3.

    Further understanding of the biological and social mechanisms linking climate change and adverse pregnancy outcomes.

  4. 4.

    Leveraging multidisciplinary and multilevel collaboration to identify and evaluate strategies to adapt to or mitigate health impacts for pregnant women

  5. 5.

    Facilitation of availability, accessibility, and timeliness of population-based environmental and health data, especially in hard-hit areas

  6. 6.

    Consensus on important study design aspects and reporting strategies that can support decision making at local, regional, national, and international level

Conclusions

Climate change is considered the biggest public health threat of the twenty-first century. Pregnant women and the growing fetus, especially those from areas with less resources, are particularly vulnerable to its direct and indirect impacts. Policy, clinical, and research strategies to adapt to or mitigate the effects of climate change are now more important than ever. A successful solution should involve close collaboration of interdisciplinary and multilevel bodies including the government, community partners, the public, physicians, industry partners, public health practitioners, and researchers.