A systematic review and meta-analysis published in Nature Medicine highlights the serious health risks associated with extreme heat exposure during pregnancy, including preterm births and hypertensive complications. This research underscores an urgent public health concern as global temperatures continue to rise.
The study, which analyzed 198 research articles from 66 countries across six continents, reveals that pregnant women are particularly vulnerable to heat stress due to metabolic changes and increased fat deposits during pregnancy. Heat stress is linked to a range of complications, such as preterm birth, stillbirth, congenital disabilities, gestational diabetes, and hypertensive disorders, as well as cardiovascular issues during labor.
Approximately 75% of the studies reviewed identified five critical health outcomes related to heat exposure: preterm birth, low birth weight, hypertensive disorders, congenital disabilities, and stillbirth. The analysis consistently found a correlation between heat exposure and an increased risk of hypertensive disorders, including pre-eclampsia and gestational hypertension, along with gestational diabetes.
Additional maternal health risks associated with heat exposure included a higher incidence of antenatal bleeding, placental abruption, pre-labor rupture of membranes, and bacterial infections. Notably, only two studies examined the impact of heat on maternal mental health, revealing a U-shaped relationship between temperature and emotional stress. The review also noted increased risks of cesarean deliveries and cardiovascular events in the week prior to childbirth.
The study identified stillbirth and congenital disabilities as the most common fetal health outcomes linked to heat exposure. One meta-analysis indicated a 1.14-fold increase in the risk of stillbirth for each 1°C rise in temperature. While most studies suggested that heat exposure during early pregnancy heightens the risk of congenital disabilities, some reported a protective effect.
Other fetal health concerns associated with heat included non-reassuring fetal status, which encompasses fetal hypoxia and growth restriction, as well as spontaneous abortion. One study documented a 50% increase in the risk of perinatal death attributable to extreme heat exposure.
Preterm birth and low birth weight emerged as the primary health outcomes for newborns related to heat exposure, both of which are critical determinants of child mortality and overall health. The meta-analysis revealed a 1.04-fold increase in the risk of preterm birth for every 1°C rise in temperature, with a heightened risk of 1.26 during heatwaves. The highest risks for heat-related preterm births were observed in low-income countries compared to upper-middle and high-income nations.
Regarding low birth weight, the analysis found a 1.29-fold increase in risk associated with higher heat exposure. A significant U.S.-based study involving 34.7 million births indicated that each additional day with mean temperatures between 26.7-32.2°C in the previous nine months was linked to an increased risk of very low birth weight, particularly among Black and Hispanic mothers.
The researchers categorized their findings into five composite outcome groups: pregnancy-specific medical disorders (including gestational diabetes and hypertensive disorders), obstetric complications (such as antenatal bleeding and preterm birth), pregnancy loss (including spontaneous abortion and stillbirth), fetal growth effects (like growth restriction and low birth weight), and healthcare system burdens (including cesarean sections and maternal and newborn admissions).
The risk of obstetric complications was found to increase by 1.05 for every 1°C rise in heat exposure, and by 1.25 during heatwaves. Due to high variability among studies, other composite outcomes were not included in the meta-analysis.
Conclusion
This comprehensive analysis underscores the significant impact of heat exposure on maternal and newborn health, emphasizing the need for prioritizing these issues within national climate and health programs. The pathophysiological mechanisms behind these health adversities include elevated maternal body temperature, dehydration, electrolyte imbalances, endocrine dysfunction, altered glucose metabolism, and hormonal imbalances. Additionally, increased levels of heat shock proteins due to heat exposure may contribute to premature labor and placental insufficiency through heightened cytokine release and tissue inflammation.
As climate change continues to escalate, addressing these health risks is crucial for protecting vulnerable populations, particularly pregnant women and their newborns.
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