Heat Exposure Risks for Maternal & Newborn Health

Extreme heat poses serious health risks during pregnancy, including risks of premature birth and hypertensive disorders. This issue is becoming a significant public health concern as global temperatures rise.

A recent study in Nature Medicine summarizes how heat exposure affects maternal, fetal, and infant health.

Introduction

Rising global temperatures and more frequent extreme weather events increase health risks, particularly for vulnerable groups like pregnant women and newborns. Pregnant women are especially vulnerable to heat stress because of added heat from fetal metabolism and increased body fat. Heat exposure during pregnancy has been linked to preterm birth, stillbirth, birth defects, gestational diabetes, hypertension, and heart complications during labor.

This comprehensive review examined 198 studies from 66 countries, most of which were conducted in high-income regions with temperate climates.

Maternal Health Outcomes

Heat exposure was consistently linked to a higher risk of hypertensive disorders, such as pre-eclampsia, gestational hypertension, and gestational diabetes. Other heat-related risks included increased chances of prenatal bleeding, placental abruption, pre-labor membrane rupture, and bacterial infections. Two studies noted a link between heat exposure and cesarean delivery and cardiovascular events just before labor. Only two studies examined heat’s impact on mental health, finding a U-shaped relationship with emotional stress.

Fetal Health Outcomes

The most common fetal risks related to heat exposure included stillbirth and birth defects. A meta-analysis found a 1.14 times higher risk of stillbirth for every 1°C rise in temperature. Research suggested that heat exposure in early pregnancy could raise the chance of congenital disabilities, although some studies reported protective effects. Other fetal risks included poor fetal condition, such as hypoxia, growth restriction, and spontaneous abortion. One study found that high heat exposure increased fetal death risk by 50%.

Newborn Health Outcomes

Heat exposure was associated with preterm birth and low birth weight, both major predictors of child mortality and health. Research across different regions showed a dose-response link between heat and preterm birth. A meta-analysis found a 1.04-fold increase in preterm birth risk per 1°C rise and a 1.26-fold increase during heatwaves. Low-income countries experienced higher rates of heat-related preterm births than middle- and high-income countries.

Regarding low birth weight, risk increased by 1.29 with higher heat exposure. A large U.S. study found that each additional hot day during pregnancy increased the risk of very low birth weight, particularly among Black and Hispanic mothers. Other heat-related newborn outcomes included low gestational age, hospitalizations, illness, and mortality.

Maternal Health – Composite Outcomes

The researchers grouped heat-related risks into five main categories: medical disorders (e.g., gestational diabetes, hypertensive disorders), obstetric complications (e.g., antenatal bleeding, preterm birth), pregnancy loss (e.g., spontaneous abortion, stillbirth), fetal growth effects (e.g., growth restriction, low birth weight), and healthcare burdens like higher cesarean rates. Obstetric risks increased by 1.05 per 1°C rise and 1.25 during heatwaves. Other categories were excluded from the meta-analysis due to variability.

Significance

Heat exposure during pregnancy significantly increases health risks for mothers and infants, highlighting the need to prioritize maternal and infant health in climate strategies. Heat exposure can raise maternal body temperature, cause dehydration, and disrupt endocrine functions. It may also trigger overexpression of heat shock proteins, increasing cytokine release and inflammation, which can contribute to preterm labor and placental problems.

For more information: Lakhoo, D. P., Brink, N., Radebe, L., et al. (2024). A systematic review and meta-analysis of heat exposure impacts on maternal, fetal and neonatal health. ß. doi:10.1038/s41591-024-03395-8