A new study published in Nature Medicine reveals that exposure to extreme heat during pregnancy significantly increases the risks of adverse health outcomes for both mothers and newborns. This research underscores the urgent public health implications of rising global temperatures and their impact on vulnerable populations, particularly pregnant women.
The systematic review analyzed data from 198 studies conducted across 66 countries and six continents, predominantly in high-income nations and temperate climates. The findings highlight that heat stress during pregnancy is linked to a range of complications, including preterm births, stillbirths, congenital disabilities, gestational diabetes, hypertensive disorders, and cardiovascular issues during labor.
Approximately 75% of the studies reviewed identified five critical maternal health outcomes associated with heat exposure: preterm birth, low birth weight, hypertensive disorders, congenital disabilities, and stillbirth. The review consistently found that heat exposure correlates with an increased risk of hypertensive disorders such as pre-eclampsia and gestational hypertension, as well as gestational diabetes.
Additional maternal health concerns linked to heat include higher rates of antenatal bleeding, placental abruption, pre-labor rupture of membranes, and bacterial infections. Only two studies examined the impact of heat on maternal mental health, revealing a U-shaped relationship between temperature and emotional stress. Furthermore, two studies indicated increased risks of cesarean deliveries and cardiovascular events in the week leading up to childbirth.
The review reported stillbirth and congenital disabilities as the most common fetal health issues related to heat exposure. A meta-analysis indicated that for every 1°C increase in temperature, the risk of stillbirth rises by 14%. Many studies focused on the critical early weeks of pregnancy, where heat exposure was linked to increased risks of congenital disabilities, although some studies noted a protective effect in certain contexts.
Other fetal health risks associated with heat include non-reassuring fetal status, fetal hypoxia, growth restrictions, and spontaneous abortion. One study reported a 50% increase in perinatal death linked to extreme heat exposure.
Preterm birth and low birth weight emerged as the most frequently reported newborn outcomes in relation to heat exposure, both of which are significant determinants of child mortality and overall health. The meta-analysis found a 1.04-fold increase in the risk of preterm birth for each 1°C rise in temperature, with a more pronounced 1.26 increase during heatwaves. Notably, the highest risks were observed in low-income countries compared to upper-middle and high-income nations.
For low birth weight, the analysis indicated a 1.29 increased risk associated with higher heat exposure. A significant U.S.-based study involving 34.7 million births found that each additional day with mean temperatures between 26.7-32.2°C in the preceding nine months corresponded to a 0.008 per 1,000 increase in the risk of very low birth weight, particularly among Black and Hispanic mothers.
Other heat-related outcomes for newborns included small-for-gestational-age births, neonatal admissions, morbidity, and mortality.
The researchers developed five composite outcome groups based on their findings, encompassing pregnancy-specific medical disorders, obstetric complications, pregnancy loss, fetal growth effects, and healthcare system burdens related to cesarean sections and maternal and newborn admissions. The analysis revealed that the risk of obstetric complications increased by 1.05 for every 1°C rise in temperature and by 1.25 during heatwaves.
Conclusion
The study highlights that heat exposure during pregnancy can significantly elevate the risks of health complications for mothers and newborns, emphasizing the need for enhanced maternal and newborn health considerations within national climate-health initiatives. Existing evidence suggests that heat exposure can lead to increased maternal body temperature, dehydration, electrolyte imbalances, endocrine dysfunction, altered glucose metabolism, and hormonal disruptions. Additionally, the upregulation of heat shock proteins may contribute to premature labor and placental insufficiency through increased cytokine release and tissue inflammation.
These findings call for urgent action to address the intersection of climate change and maternal health, ensuring that vulnerable populations receive the necessary support to mitigate these risks.
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