Extreme heat is emerging as one of the most consequential—and least clearly defined—threats to pregnancy. A new systematic review and meta-analysis published in Nature Communications examines the temperatures at which heat exposure begins to affect maternal health and birth outcomes, bringing together a fragmented body of research from different countries, climates, and healthcare systems. The study, led by C.S. Heil, L.G. Ioannou, G. Alce and colleagues, focuses on a question that sounds simple but has proven scientifically difficult: how hot is too hot for pregnant people and developing babies?
The answer is not a single number. Heat risk depends on more than the air temperature reported by a weather station. Humidity, wind speed, solar radiation, nighttime cooling, duration of exposure, physical activity, housing conditions, access to air conditioning, and individual health all influence how much thermal stress the body experiences. Pregnancy adds another layer of complexity. As the fetus grows, the body must regulate its own temperature while supporting increased blood flow, cardiovascular demand, and metabolic activity. These changes can reduce the physiological reserve available during periods of intense heat.
The review addresses this complexity by examining evidence linking heat exposure with a broad range of maternal and neonatal outcomes. These include heat-related illness, hospital admissions, hypertensive disorders of pregnancy, gestational diabetes, preterm birth, low birth weight, fetal growth restriction, stillbirth, and other indicators of adverse birth outcomes. Some of these outcomes can develop rapidly during heatwaves, while others may reflect cumulative exposure over days or weeks. By combining findings across studies, the researchers aim to identify patterns that may be invisible in individual investigations.
A central challenge is that researchers do not measure heat in the same way. Some studies use daily maximum temperature, while others analyze average temperature, apparent temperature, heat index, or wet-bulb globe temperature. Apparent temperature combines air temperature and humidity to estimate how hot conditions feel to the human body. Wet-bulb globe temperature also incorporates radiant heat and wind, making it more relevant to physiological heat strain in certain settings. Other studies classify exposure according to local percentiles, such as temperatures above the 90th or 95th percentile, rather than using a fixed Celsius or Fahrenheit threshold.
This distinction matters because a temperature that is dangerous in one region may be relatively ordinary in another. Populations adapt to their local climate, and buildings, public-health systems, work practices, and social conditions can shape vulnerability. A moderate heat increase in a humid tropical city may impose a greater physiological burden than a higher dry-bulb temperature in an arid region. The review therefore highlights the difficulty of translating epidemiological findings into universal public-health warnings. Thresholds based on relative local temperature may capture unusual heat more effectively, while absolute thresholds may be easier to communicate across borders.
The biological mechanisms connecting heat with pregnancy outcomes are increasingly understood. When the body becomes overheated, blood vessels near the skin widen to release heat, and sweating increases fluid loss. During pregnancy, this redistribution of blood flow may place additional demands on the cardiovascular system and could affect circulation to the placenta. Dehydration can reduce plasma volume and intensify cardiovascular strain. Heat stress may also stimulate inflammatory and hormonal pathways associated with uterine activity. These mechanisms do not mean that every hot day will trigger a complication, but they help explain why sustained or extreme heat can influence both maternal well-being and fetal development.
For the fetus, the timing of exposure may be critical. Early pregnancy involves rapid cellular differentiation and formation of major organs, while later pregnancy is characterized by accelerated growth and increasing demands on the placenta. Heat exposure during different gestational windows may therefore be associated with different outcomes. A short period of extreme heat close to delivery could influence the risk of preterm birth, whereas repeated exposure earlier in pregnancy might affect fetal growth or development. The review’s broad design allows researchers to compare these time windows and assess whether the evidence points to particularly sensitive stages.
The study also places heat risk in a social context. Pregnant people who work outdoors, live in poorly ventilated housing, lack reliable electricity, or cannot access cooling may face far higher exposure than temperature records suggest. Urban heat islands can raise nighttime temperatures, limiting the body’s opportunity to recover after a hot day. Household responsibilities, transportation barriers, poverty, and limited access to prenatal care can further restrict the ability to respond to heat warnings. These factors mean that a temperature threshold should not be interpreted as a biological boundary affecting everyone equally; it is better understood as an indicator of rising population risk.
By systematically comparing previous research, the authors address another important problem: studies often reach different conclusions because they use different designs, populations, exposure windows, and statistical models. Meta-analysis can estimate overall patterns, but it must also account for heterogeneity—the degree to which results vary between studies. Researchers may examine whether differences arise from climate zone, season, maternal characteristics, gestational age, or the way heat is defined. They must also consider confounding factors such as air pollution, seasonal changes in infectious disease, access to medical care, and socioeconomic conditions. These technical choices determine how confidently evidence can be translated into clinical or policy recommendations.
The findings arrive as climate change is making extreme heat more frequent, longer-lasting, and more widespread. Global warming does not simply add a few hotter days to the calendar; it shifts the entire distribution of temperatures and increases the likelihood of compound events, including heat combined with drought, wildfire smoke, or power outages. Pregnancy care is consequently becoming part of climate adaptation. Health authorities may need heat-alert systems specifically designed for pregnant people, guidance for clinicians, workplace protections, cooling access, and public messaging that recognizes humidity and nighttime heat rather than relying on daytime temperature alone.
The review’s importance lies not only in identifying harmful temperatures but also in showing why a universal “safe” threshold is difficult to establish. Risk generally rises across a continuum, and vulnerable individuals may experience harm before an official heatwave declaration is issued. A threshold used for public-health action is therefore different from a precise biological cutoff. The former is a practical trigger intended to prevent illness; the latter would imply a sharp boundary that the available evidence may not support. Distinguishing between these concepts could help policymakers design earlier and more targeted interventions.
As the climate continues to warm, the question of heat and pregnancy will become increasingly urgent. The systematic review provides a framework for interpreting evidence that has often been scattered across disciplines and regions. It also underscores the need for more standardized heat measurements, better reporting of pregnancy stage and individual vulnerability, and studies in low- and middle-income settings where exposure and healthcare inequalities can be greatest. For clinicians and families, the message is clear even when the precise threshold is not: extreme and persistent heat should be treated as a serious environmental health stressor during pregnancy, not merely as an inconvenience of summer.
Subject of Research: Heat exposure thresholds affecting maternal health and birth outcomes during pregnancy.
Article Title: A systematic review and meta-analysis on heat thresholds for maternal health and birth outcomes
Article References: Heil, C.S., Ioannou, L.G., Alce, G. et al. A systematic review and meta-analysis on heat thresholds for maternal health and birth outcomes. Nat Commun 17, 8768 (2026). https://doi.org/10.1038/s41467-026-76822-8
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41467-026-76822-8
Keywords: extreme heat, pregnancy, maternal health, birth outcomes, preterm birth, heatwaves, climate change, fetal health, systematic review, meta-analysis
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