Heat exposure significantly heightens risks for maternal and newborn health

Exposure to extreme heat may pose serious risks during pregnancy, from preterm births to hypertensive complications, highlighting an urgent public health issue amid rising global temperatures.

Study: A systematic review and meta-analysis of heat exposure impacts on maternal, fetal and neonatal health. Image Credit: Shutterstock AI / Shutterstock.com

A recent study published in the journal Nature Medicine provides a detailed overview of the impact of heat exposure on maternal, fetal, and newborn health. 

Introduction

The global rise in environmental temperature, combined with the increasing frequency of extreme weather events, is becoming a major public health concern, especially for vulnerable populations, including pregnant women and newborns.

Heat generated by fetal metabolism and additional fat deposits during pregnancy makes pregnant women more susceptible to heat stress. Heat stress during pregnancy is associated with a wide range of complications, including preterm birth, stillbirth, congenital disabilities, gestational diabetes, hypertensive disorders, and cardiovascular events during labor.

The current systematic review included 198 studies from 66 countries conducted across six continents, most of which were performed in high-income countries and temperate climate zones.

Maternal health outcomes

About 75% of the selected studies highlighted five critical health outcomes related to heat exposure, including preterm birth, low birth weight, hypertensive disorders during pregnancy, congenital disabilities, and stillbirth.

Heat exposure was consistently associated with an increased risk of hypertensive disorders such as pre-eclampsia and gestational hypertension, as well as gestational diabetes in pregnant women. Other heat-related maternal health outcomes included increased risks of all-cause antenatal bleeding, placental abruption, pre-labor rupture of membranes, and bacterial infections. 

Only two studies reported the impact of heat exposure on maternal mental health, in which a U-shaped association was observed between temperature and emotional stress. Heat exposure-related increased risks of cesarean section delivery and cardiovascular events during the week before childbirth were also reported by two studies.

Fetal health outcomes

Stillbirth and congenital disabilities were the most frequently reported heat exposure-related fetal outcomes. Moreover, one meta-analysis of five studies reported a 1.14 increased risk of stillbirth for every 1°C increase in heat exposure.

Most studies investigating congenital disabilities reported that heat exposure during the first few weeks of pregnancy increases the risk of these defects. However, several studies highlighted a protective association between heat exposure and the risk of congenital disabilities.

Other heat-related fetal outcomes included non-reassuring fetal status, which includes fetal hypoxia and fetal growth restriction, as well as spontaneous abortion. A 50% increased risk of perinatal death due to extreme heat exposure was reported by one study.

Newborn health outcomes

Preterm birth and low birth weight were the most frequently addressed heat-related newborn outcomes in the selected studies, both of which are major determinants of child mortality, health, and well-being. Overall, a large body of evidence across diverse geographical locations and populations established a dose-response relationship between heat exposure and preterm birth.

The current meta-analysis revealed a 1.04-fold increased risk of preterm birth for every 1°C increase in heat exposure. Considering heatwave exposure, the risk of preterm birth increased by 1.26 in the meta-analysis. The highest risk of heat-related preterm birth was observed in low-income countries compared to upper-middle and high-income countries.

Regarding low birth weight, a 1.29 increased risk at higher versus lower heat exposure was observed. In this context, the largest United States-based study involving 34.7 million births reported that for every additional day with mean temperatures between 26.7-32.2°C in the preceding nine months was associated with an increased risk of very low birth weight by 0.008 per 1,000, particularly among Black and Hispanic mothers.

Other heat-related newborn outcomes included small-for-gestational-age, neonatal admissions, neonatal morbidity, and neonatal death.

Composite outcomes

Based on these findings, the current study’s researchers devised five composite outcome groups. These groups included pregnancy-specific medical disorders such as gestational diabetes, hypertensive disorders, and cardiovascular events; obstetric complications like antenatal bleeding, preterm birth, and pre-labor membrane rupture; pregnancy loss including spontaneous abortion and stillbirth, fetal growth effects like growth restriction, small-for-gestational-age, and low birth weight, as well as healthcare system burden based on caesarian section, as well as maternal and newborn admissions.

The risk of obstetric complications increased by 1.05 for every 1°C increase in heat exposure and by 1.25 during heat waves. Due to the high-level heterogeneity, other composite outcomes were not included in the meta-analysis.

Significance

Heat exposure during pregnancy can significantly increase the risk of health adversities in pregnant women and newborns. These findings emphasize the need to prioritize maternal and newborn health in national climate-health programs.

Regarding pathophysiological mechanisms associated with health adversities, existing evidence indicates that heat exposure during pregnancy can lead to increased maternal body temperature, maternal dehydration and electrolyte imbalance, endocrine system dysfunction, altered glucose metabolism, and hormonal imbalance. An upregulation of heat shock proteins due to heat exposure may also contribute to premature labor onset and placental insufficiency by increasing cytokine release and tissue inflammation.