Press release
A new study reveals how environmental heat exposure impacts pregnant women and unborn babies, shedding light on the biological and physiological processes that may affect pregnancy outcomes as global temperatures rise.
This systematic review and meta-analysis bring together decades of global evidence to better understand how heat stress impacts maternal, placental and fetal physiology at a time when rising temperatures are increasingly recognised as a major threat to vulnerable populations.
Pregnancy involves complex physiological adaptations that affect how the body regulates temperature and responds to environmental stress, making pregnant women particularly susceptible to extreme heat. To address this, researchers analysed data from 27 studies involving more than 201,000 pregnant women across a wide range of climatic settings, spanning both laboratory-controlled and real-world environments between 1961 and 2024.
The findings published in The Lancet Planetary Health shows consistent evidence that heat exposure triggers measurable physiological stress responses in both mothers and their fetuses. Maternal heart rate increases significantly alongside a rise in core body temperature, while skin temperature also elevates as the body responds to external heat. At the same time, fetal heart rate increases, indicating that the effects of heat exposure extend beyond the mother to directly impact the fetus. These responses, collectively described as maternal heat strain, were observed in both controlled environments such as sauna-based studies, and real-world settings, reinforcing the direct effect of environmental heat. In real-world settings, where physical activity contributes additional internal heat load, the overall impact is likely to be even greater.
The study also highlights that not all pregnancies are affected equally. Women with hypertensive disorders, particularly pre-eclampsia, appear to be more vulnerable to heat exposure. In these cases, there is a higher susceptibility to impaired blood flow between the uterus and placenta, a critical pathway for delivering oxygen and nutrients to the fetus. Evidence of fetal distress was also observed, particularly among higher-risk pregnancies, suggesting that heat stress may worsen existing conditions and increase the likelihood of adverse outcomes.
Dr Ana Bonell, Assistant Professor at MRC Unit The Gambia at LSHTM, said: “This review confirms heat exposure puts strain on both mothers and their unborn children. However, it also highlights the huge gaps in our understanding of other potential pathways that may be involved, such as inflammation, hormone changes, and others. There is a clear need to understand this better, in order to identify those women most at risk and to give evidence-based advice on how to avoid the complications we know occur when pregnant women are exposed to these high temperatures.”
While the study provides evidence of immediate physiological responses, it also highlights significant gaps in understanding the longer-term biological consequences of heat exposure during pregnancy. Many of the proposed pathways, including chronic inflammation, oxidative stress and placental dysfunction, remain underexplored. The findings align with the Developmental Origins of Health and Disease, which suggests that environmental exposures during pregnancy can influence long-term health by altering gene expression and biological development. This raise concerns that rising temperatures may not only affect pregnancy outcomes in the short term but could also shape the health of future generations.
Despite the global relevance of this issue, most studies included in the review were conducted in high-income countries, with limited data from low- and middle-income settings, including The Gambia, where heat exposure is often more intense and health systems face additional challenges. This highlights a critical gap in the evidence, particularly given that these regions are among the most vulnerable to climate-related health risks.
The researchers emphasised that while many heat-related risks are preventable, inequalities in access to healthcare, resources and information continue to deepen global disparities. Addressing these challenges will require more standardised approaches to measuring heat exposure, improved reporting of biological outcomes, and long-term research to better understand how repeated or prolonged exposure affects pregnancy and fetal development.
Overall, the study provides compelling evidence that heat exposure places significant burden on pregnant women and their babies, particularly among those with existing health conditions such as hypertension. As climate change accelerates, closing these knowledge gaps will be essential to safeguarding maternal and child health and ensuring that progress in reducing global health inequalities is not reversed.
