A Comprehensive Review on Hot Ambient Temperature and its Impacts on Adverse Pregnancy Outcomes

To find relevant articles, we utilised the following search criteria: AND pregnancy AND human beings AND temperature (OR ambient temperature OR high temperature OR hot temperature OR heat wave OR climatic change) (OR pregnancy outcome OR spontaneous abortions OR miscarriage OR gestational age OR Stillbirth OR birth weight OR preterm birth). This review examined papers on reproductive health, maternal health, birth outcomes, working women, and extreme heat. The impact of high ambient temperatures and heat waves on maternal and fetal health is the subject of this study. We eliminated research papers that did not include human population studies or dealt with topics unrelated to heat stress. The research articles were evaluated using the following criteria: exposure population, study type, location, type of exposure, measurable result, and potential sources of bias. For our review, we looked at the following definitions: The termination of a pregnancy before the 20^th week of pregnancy or the delivery of a baby weighing less than 500 grams is considered an abortion [23,24,25]. When a baby is born before the 37^th week of pregnancy, it is known as preterm birth (PTB) [22]. The World Health Organization (WHO) defines low birthweight (LBW) as a birth weight of less than 2500 grams at term (5.5 lb) and stillbirth as the birth of a baby who shows no signs of life at or after 28 weeks of pregnancy [26–27]. Following that, data extraction for the review was completed.

To determine exposure, we obtained data on temperature fluctuations and the length of heat episodes. Studies were included and examined for eligibility if they met the PICO components: population, intervention, comparator, and outcomes. We treated pregnant women and newborns as the target population; exposure to high temperatures as the intervention; hot and cold seasons as the comparator; and unfavourable maternal and fetal outcomes such as miscarriages, PTB, LBW, and stillbirths as the outcomes.

For each category of health outcome, we acquired statistical data, risk estimates, and confidence levels where available. Confounding data was also obtained, such as socioeconomic characteristics, diet, and air pollution. Each article was evaluated for biases such as selection, misclassification, detection, and internal validity. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) criteria were used to select studies [28]. Only 23 publications were found to match the review's final selection criteria, out of a total of 1500. Endnote was used to export all of the search results. The paper's title, abstract, and full text content were all examined first. We found both review papers and original research publications as a result of our search; however, we did not include review articles in our analysis. As a result, the final selection of articles was determined based on the search keywords, the reference lists of the retrieved articles for cross-referencing, and related citations from the published literature.

Pregnant women can work throughout their pregnancy for a variety of reasons, including financial necessity, insurance preservation, postpartum leave time, and job security [31]. According to Palmer et al. [32], workplace characteristics such as work hours, shift work, lifting, standing, and physical workload have a substantial impact on spontaneous abortion [33], preterm delivery, low birthweight, small for gestational age (SGA), preeclampsia, and gestational hypertension [32]. Lifting items weighing over 20 kg at work more than ten times a day poses a greater risk of premature birth for working pregnant women [34]. Even if it lowers their productivity and daily wages/piece rates because of missed work targets, heavy lifting and greater heat exposure have a detrimental effect on the physical health of female brick workers [35]. During pregnancy, dehydration caused by physical exercise or occupational heat stress may cause a minor increase in uterine contractions [36]. Heat stress causes the body to lose water, which causes the uterus to contract by releasing antidiuretic hormone and oxytocin [37].

Heat is teratogenic at all stages of development, which has substantial implications for maternal health, as it affects placental blood flow and increases the risk of hypertensive crises and stillbirth by increasing vasoactive substance production, blood viscosity, and endothelial cell activity [38]. Internal and external sources of heat, such as hot tubs, saunas, and electric blankets, can cause fevers ranging from 37.8°C to over 38.9°C that can even reach 43°C [39]. Elevated ambient temperature has been shown to have a negative influence on the reproductive function [40–41] and may result in a longer gestation time [42] during the early and late stages of embryonic development. Long-term exposure to heat beyond the threshold levels has been linked to heat strain symptoms in workers [43], and pregnant women and those with co-morbidities are particularly sensitive to heat-related illnesses (HRIs) [44]. In addition, the manner in which women deal with gynaecological issues under heat stress may have a different effect on pregnancy compared to normal conditions [45]. When working in extreme heat, pregnant women are more susceptible to dizziness, fainting, nausea and vomiting, febrile chills, migraines, and sunstroke, as well as aggravation of pre-existing low or high blood pressure disorders. Increased sensitivity to fainting, which is a serious concern during the first trimester [46], may be caused by changes in hormone levels, impaired adaptive ability, maternal overheating, and the growing circular demand of the fetus on the mother [46]. Preeclampsia, gestational hypertension, and poor newborn outcomes have all been associated with high ambient temperatures during pregnancy [47]. Warmth increases the risk of preeclampsia in the third trimester by raising metabolic load and modifying maternal physiology, limiting the fetus’ ability to cope with the expanding fetus's physiological needs [48]. During the early and late phases of embryonic development, elevated ambient temperature has been found to have a deleterious impact on reproductive function [40] and may result in a longer gestation time [42]. In 2005, Motherisk published a systematic review and meta-analysis on maternal hyperthermia during the first trimester and the risk of Neural Tube Defects (NTDs) in humans [49].

Heat stress during the first months of pregnancy increases the likelihood of congenital deformities, especially of the central nervous system, spontaneous abortions, gestational age, or mental health anomaly, or stillbirth compared to exposures in the second or third trimesters [50,51,52,53]. Although certain studies found no associations between heat and preeclampsia, delayed labour, or antepartum or postpartum haemorrhage [54–55], pregnant women experienced more Emergency Room (ER) visits during heat waves due to an increase in cardiovascular events such as stroke and myocardial infarction [56–57]. Additionally, stress and sleep problems have also been linked to high temperatures in pregnant women [58–59]. DNA cleavage, chromatin clustering, disrupted mitosis, cytoskeleton modification, and altered circulation increase teratogenicity in pregnant women exposed to high temperatures [17]. Abortions, stillbirths, preterm deliveries, abnormalities, and intrauterine growth restriction are only a few of the prevalent obstetric disorders known as APOs that are much more common in developing nations [21]. Working women in low and middle-income nations, particularly those working outside in tropical countries, face significant levels of physical effort and heat exposure [46]. Women continue to work until childbirth for a variety of reasons, despite the fact that they are not allowed to work for more than two hours beyond their permissible working hours in an unduly hot atmosphere [32].

Environmental factors are associated not only with pregnancy-related outcomes but also with birth outcomes such as congenital cataract, low birthweight, and early delivery [30]. Pregnant women are not only impacted by heat, but their newborns are also at risk of developing heat-related diseases such as rashes [60]. Strand et al. examined seasonal patterns in birth outcomes as a function of ambient temperature and found significant associations with low birthweight, preterm birth, and stillbirth [61] independent of maternal ethnicity or age, and younger women are at a significantly increased risk for adverse outcomes [38, 61,62,63]. Extreme heat events worsen the incidence of congenital cardiac defects, as predicted by maximum temperatures trends [64]. Intrauterine growth retardation can occur at multiple stages of gestation, with mid- to late-stage gestational temperatures having the greatest impact [65]. The Aberdeen study indicated that mothers exposed to high ambient temperatures during the first trimester of pregnancy gave birth to infants with LBW [66]. Birth abnormalities occur when a woman is exposed to heat for the bulk of her gestational weeks [8,9,10,11, 67–68], including preterm birth, low birth weight, and infant death [16, 69,70,71,72,73,74,75]. Heat-induced dehydration can lead to overheating, changes in the structure of the fetal heart, and neural tube abnormalities (48), which can lead to disorders such as spina bifida, which has been linked to a high fever or hot tub use, especially during the first trimester (American Association News). The number of days fetuses were exposed to maximum daily temperatures of 30 °C was associated with an increased frequency of congenital heart defects in the newborn population [76], including non-critical defects such as an atrial septal defect [8]. Children delivered in heat-affected regions are more likely to suffer from at least one of a group of abnormal delivery disorders (including fetal discomfort, assisted ventilation for more than 30 minutes, and meconium aspiration syndrome) [77]. In LMICs, women from lower socioeconomic groups were more likely to give birth prematurely, with rates of 10.4% in Asia and 12.0% in Sub-Saharan Africa [78]. These young women are compelled to take low-paying jobs outside the home in order to sustain their families [79].

Pregnant women who work are more susceptible to heat stroke and exhaustion because they must support the additional weight of the fetus while cooling their own and the fetus's bodies. Working outdoors during pregnancy increases the risk of heat-related diseases, which might affect the unborn child due to elevated body temperature [80]. Heat exacerbates the hazards of outdoor activities during pregnancy, including brickmaking, agriculture, and salt pans. APOs, including spontaneous abortions, are associated with hazardous chemicals and physical agents (high heat and strenuous labour), as well as ergonomic risks [81]. This review will enable researchers to develop individual and community-level strategies to protect pregnant women from heat stress, such as public health interventions and protective labour policies, by enhancing their understanding of the plight of pregnant women who are frequently exposed to heat stress, which can then be scaled up to other similar settings elsewhere.