In a fictional case, a 27-year-old woman, 34 weeks pregnant, sells fruit at an open-air market. The city is on its fourth consecutive day of unusual heat, and her house, with a metal roof and one small window, stays hot at night. She arrives at her prenatal visit with a headache, thirst and irregular contractions since the day before. Her question is simple: can the heat make the baby be born early?

The short answer is yes. The World Health Organization (WHO), UNICEF and UNFPA state that extreme heat is associated with preterm birth, low birth weight, stillbirth, gestational diabetes and hypertensive disorders of pregnancy (WHO, UNICEF and UNFPA, 2023). The more useful question for those providing care is another one: who is most exposed, at what point in pregnancy, and what can be advised at the visit.

This text uses WHO publications as its backbone and brings together recent systematic reviews on heat and pregnancy, with special attention to low- and middle-income countries, where exposure is greater and data are scarcer.

Key points

• In a meta-analysis with studies from 27 countries, the chance of preterm birth rose 5 percent for each 1 °C increase in temperature and 16 percent during heatwaves (Chersich et al., 2020).
• The risk of stillbirth also rose by about 5 percent per degree, according to the same meta-analysis (Chersich et al., 2020).
• The final weeks of pregnancy are a sensitive window, and the effect appears within a few days after exposure (Chersich et al., 2020; Fitch et al., 2026).
• Low-income women, who work outdoors or live in hot houses, carry most of the risk (WHO, descriptive note).
• WHO guidance for the public, with shade, ventilation, water and attention to official alerts, can be brought into prenatal care (WHO, descriptive note).

• Why pregnant women tolerate heat worse
• What the reviews show about preterm birth
• Stillbirth, low birth weight and other outcomes
• Who is most exposed
• Exposure windows and temperature thresholds
• What to advise in prenatal care
• Health services and heat alerts
• How NEXO helps train this reasoning
• Frequently asked questions
• Take-home messages

Why pregnant women tolerate heat worse

Pregnancy changes the way the body produces and loses heat. The metabolism of the fetus and the placenta generates extra heat, and this heat must be dissipated by the mother (Chersich et al., 2020). The more advanced the pregnancy, the greater the mass to cool and the smaller the margin for facing a hot environment.

The reviews describe some plausible pathways between heat and poor outcomes. A scoping review summarizes three of them: hormonal change, with increased cortisol; dehydration; and diversion of blood flow to the skin, away from the placenta and the fetus (Brimicombe et al., 2024). Another review counted which mechanisms the study authors cited most often and found inflammation, dehydration, circulatory changes with reduced uteroplacental flow and failure of thermoregulation (Holvoet et al., 2026).

These mechanisms are still hypotheses, because few studies have directly measured what happens in a pregnant woman's body during a heatwave. The authors of the scoping review note that the biological pathways remain poorly understood and call for primary research (Holvoet et al., 2026). For clinical practice, this means that guidance rests on consistent associations in large populations, not on an individual marker that says who will have a preterm birth.

There is also a practical point. Signs of heat strain, such as heavy sweating, increased body temperature, dizziness, headache and cramps, appear with shorter exposures and lower temperatures than the effects on birth (Heil et al., 2026, Nat Commun). These symptoms are therefore a warning sign that arrives before the obstetric outcome.

What the reviews show about preterm birth

The most cited meta-analysis gathered 70 studies from 27 countries and showed a consistent association between high temperatures and preterm birth (Chersich et al., 2020). The odds ratio was 1.05 per degree increase in temperature and 1.16 during heatwaves, with a greater effect the more intense and longer the heatwave (Chersich et al., 2020).

Later studies repeated the finding in different contexts. A review with data from low- and middle-income countries found an association between heat and preterm birth in four of six studies (Lakhani et al., 2024). Another meta-analysis focused on the same countries calculated an odds ratio of 1.29 for heat exposure in the first trimester and 1.39 in the third (Raghavan et al., 2025).

In high-income countries, the effect is smaller in size, but it appears in very detailed data. A study of eight US states used temperature measured at the neighborhood scale, able to capture urban heat islands (Fitch et al., 2026). Three or four consecutive days above the 97.5th percentile of mean temperature modestly increased the chance of preterm birth and of early-term birth, between 37 and 38 weeks, in the following days (Fitch et al., 2026).

A review of reviews confirms the overall picture: preterm birth was the outcome most associated with extreme heat, present in eight reviews (Álvarez-García et al., 2026). The broadest systematic review on climate and pregnancy, with 596 publications, concluded that rising temperatures and extreme events have clear negative consequences for pregnant women and babies (Persson et al., 2025).

The numbers in perspective

An odds ratio of 1.05 seems small for one woman. Across whole populations, however, it adds up with every hot day and every pregnancy exposed. The WHO notes that heat affects millions of people at the same time and can overload health services (WHO, fact sheet).

Preterm birth is already a major problem without heat. It affects about 13.4 million babies a year worldwide, roughly one in ten births (Heil et al., 2026, Nat Commun). Any factor that shifts this rate in large populations carries public health weight.

Stillbirth, low birth weight and other outcomes

Stillbirth is the most severe outcome associated with heat. In the 2020 meta-analysis, the risk rose with an odds ratio of 1.05 per degree (Chersich et al., 2020). A scoping review concluded that the more intense and prolonged the exposure to heat stress, the greater the risk of stillbirth (Brimicombe et al., 2024).

A large national cohort in China helps to understand the shape of this relationship. Exposure to heat waves was associated with a 36 to 83 percent increase in the risk of stillbirth, depending on the wave definition used, while cold waves had no significant effect (Li et al., 2025). Regions with lower population density and more medical resources had smaller effects, and small, vulnerable newborns were the most affected (Li et al., 2025).

Low birth weight appears in a good part of the studies, but with less consistency. In the 2020 meta-analysis, weight was lower with heat in 18 of 28 studies (Chersich et al., 2020). In low- and middle-income countries, the association appeared in four of five low birth weight studies and in three of four stillbirth studies (Lakhani et al., 2024).

The list of outcomes studied is longer than the list of well-established outcomes. A 2026 meta-analysis cites preeclampsia, congenital heart defects and placental abruption (Heil et al., 2026, Nat Commun). The scoping review from the same year adds neonatal mortality and congenital anomalies (Holvoet et al., 2026), and the WHO call includes gestational diabetes and effects on the mother's mental health (WHO, UNICEF and UNFPA, 2023).

The effect of heat on gestational hypertension connects to the reasoning of another blog article on the sFlt-1/PlGF ratio in the diagnosis of preeclampsia. The effects of heat on mental health in general are in the article on heat and mental health.

Comparison between the reviews

Review	Studies	Preterm birth	Stillbirth	Low birth weight
Chersich et al., 2020, worldwide	70	OR 1.05 per °C; 1.16 in heat waves	OR 1.05 per °C	Lower weight in 18 of 28 studies
Lakhani et al., 2024, low and middle income	11	Association in 4 of 6 studies	Association in 3 of 4 studies	Association in 4 of 5 studies
Raghavan et al., 2025, low and middle income	32	OR 1.29 in the 1st trimester; 1.39 in the 3rd	Out of scope	Out of scope
Holvoet et al., 2026, worldwide	61	Outside the main focus	Consistent association	Consistent association

Table sources: Chersich et al., 2020; Lakhani et al., 2024; Raghavan et al., 2025; Holvoet et al., 2026. OR is the odds ratio. Heterogeneity between studies was very high, and the review by Lakhani and colleagues could not perform a meta-analysis.

Who is most exposed

Heat risk is distributed unevenly. The WHO points to outdoor workers and poor populations, urban and rural, as the most exposed, and notes that gender matters, for example when women cook in closed, hot environments (WHO, descriptive note). In the 2020 meta-analysis, the risk was higher in groups of lower socioeconomic status and at the extremes of maternal age (Chersich et al., 2020).

The profiles that deserve attention in the consultation include:

• pregnant women who work outdoors, in agriculture, construction or street commerce;
• pregnant women who cook on stoves indoors or work in closed environments without ventilation;
• residents of houses with metal roofs, little ventilation or without reliable access to power and water;
• very young or older pregnant women;
• pregnant women with multiple pregnancy, obesity, malaria, HIV infection, mental disorders or other chronic diseases, who may have less capacity to respond to heat (Chersich et al., 2020).

Work in the heat also has effects outside pregnancy. Kidney disease of unknown cause in rural workers is an example described in the text on kidney disease in people who work in the heat. The logic that those who need the most receive the least care, discussed in the text on the inverse care law, applies here too.

The role of air conditioning

The data on cooling help to understand the inequality. A study in Shenzhen, China, found an apparently protective effect of heat, which the authors attributed to widespread use of air conditioning, and a study in northern California had the smallest effects in areas with more air conditioning (Chersich et al., 2020). Those who can cool their home reduce part of the risk; those who depend on the outdoors or on a hot house keep all of it.

The meta-analysis also draws attention to the night. Three studies found that nighttime temperature or the variation between day and night predicted preterm birth better than daytime temperature (Chersich et al., 2020). A house that does not cool down at night takes away the pregnant woman's recovery period.

Exposure windows and temperature thresholds

The timing of exposure matters. In high-income countries, the effect on preterm birth and stillbirth was concentrated in the last weeks of pregnancy, with a short interval, from zero to three days, between the heat and delivery (Chersich et al., 2020). In low- and middle-income countries, the risk appeared throughout pregnancy, and a meta-analysis found effects in the first and third trimesters (Raghavan et al., 2025).

The most recent reviews still treat the windows as being defined. The 2026 scoping review concludes that the critical exposure windows are emerging, but have not yet been definitively established (Holvoet et al., 2026).

From how many degrees

A 2026 meta-analysis tried to answer at what maximum temperature the risk starts to rise. With 21 studies, it estimated thresholds of 30.55 °C, 34.11 °C and 39.07 °C for increases of 5, 10 and 20 percent in the risk of maternal and birth outcomes (Heil et al., 2026, Nat Commun). The authors suggest using these thresholds as three alert levels for pregnant women, but acknowledge the heterogeneity of the studies and the fact that data extraction was not done in duplicate, independently (Heil et al., 2026, Nat Commun).

Air temperature also does not tell the whole story. A scoping review found four different indices in the studies: the heat index, the apparent temperature, the wet bulb globe temperature index and the universal thermal climate index (Brimicombe et al., 2024). These indices combine humidity, wind and solar radiation, and each one can classify the same day differently.

For clinical practice, the message is practical. The exact threshold is still under discussion, and the region's official heat alerts remain the best reference for advising pregnant women, as the WHO recommends (WHO, descriptive note).

What to advise in prenatal care

WHO publishes guidance for the public during heatwaves, and it is the basis for the conversation with the pregnant woman (WHO, descriptive note). The main ones are:

• avoid going out and exerting yourself during the hottest hours of the day and seek shade;
• spend two to three hours a day in a cool place;
• follow official heat alerts;
• cool the house at night by opening windows when the outside air is cooler, and close curtains during the day;
• use a fan only when the temperature is below 40 °C, and, if there is air conditioning, set it to about 27 °C together with a fan;
• wear light, loose clothing, take cool showers and wet the skin;
• drink water regularly, about one glass per hour, and at least two to three liters a day.

Adapt to what the pregnant woman has at home

General advice needs to fit real life. A qualitative study in Kilifi, Kenya, listened to pregnant and postpartum women and described five strategies they already used: less clothing, shade, ventilation, cool baths and water, and shifting the timing of tasks (Lusambili et al., 2026). The barriers were hot houses, unstable power and lack of water, and some solutions had a cost, such as sleeping outdoors, which exposes them to mosquitoes and safety risks (Lusambili et al., 2026).

The conversation works best when it starts from what the pregnant woman describes. It is worth asking where and at what time she works and whether she can move tasks to early morning, whether the house cools at night and whether there is a cool place nearby to spend part of the day. It also helps to know whether she has drinking water at hand, whether she cooks in a closed environment and who can take over heavy tasks on alert days.

Physical activity is still recommended in most pregnancies, but on intense heat days it is worth changing the time and reducing the intensity, as discussed in the text on exercise during pregnancy.

Signs to seek care

During a heatwave, the pregnant woman should know when to seek care. Regular contractions before term, loss of fluid, bleeding, reduced baby movements, confusion, fainting or very high body temperature are reasons to go to care right away. The approach to heat illness in adults, including cooling before any investigation, is in the text on heatstroke, dehydration and medicines that make it worse and in the text on exertional heat stroke.

Reviewing medications is also part of the consultation. Diuretics and other medicines that affect hydration or thermoregulation deserve attention on a hot day, as shown in the text on extreme heat cited above.

Health services and heat alerts

The pregnant woman does not face the heat only at home. In the 2020 meta-analysis, the authors observed that health facilities can be up to 4 °C hotter than the outdoor environment (Chersich et al., 2020). They suggest ventilation, fans, cool water, heat counseling and cash transfers as measures to test (Chersich et al., 2020).

WHO notes that heat can also disrupt the functioning of health services, with more demand and infrastructure failures (WHO, descriptive note). The joint call from WHO, UNICEF and UNFPA notes that few national climate strategies mention maternal and child health and calls for surveillance, research and resilient health systems (WHO, UNICEF and UNFPA, 2023). In Europe, most heat strategies did not even list pregnant women as a risk group (Chersich et al., 2020).

Alerts by mobile phone

Early warning systems are one of the bets. A study in South Africa evaluated a heat alert app with 100 pregnant women and 101 postpartum women (Heil et al., 2026, JMIR). Users found the app easy to use and reported having learned about heat, but protective behaviors decreased over the course of the study, and participant loss reached 30 percent among pregnant women (Heil et al., 2026, JMIR).

The result suggests that the alert alone is not enough. The authors of the Kilifi study conclude that guidance on heat should be part of prenatal and postpartum routine care (Lusambili et al., 2026). The review of reviews reaches a similar conclusion and highlights the role of health professionals in counseling (Álvarez-García et al., 2026).

Context also matters for maternal mortality in general. Delays in deciding to seek help, in reaching the service and in receiving adequate care, described in the text about the three delays model, tend to grow when heat overloads families, transport and hospitals.

Back to the case

The street vendor from the beginning is in her third trimester, works outdoors and lives in a house that does not cool down at night. According to the reviews, she has several higher-risk factors: late pregnancy, prolonged exposure and little possibility of cooling (Chersich et al., 2020).

The consultation must first rule out preterm labor and heat illness, with assessment of contractions, the cervix, fetal wellbeing, blood pressure and hydration. Then comes concrete guidance: work in the shade and early in the morning, drink water throughout the day, have a cool place to rest and follow official alerts, as the WHO advises (WHO, fact sheet). She should also leave the consultation knowing which signs require returning to the service the same day.

How NEXO helps train this reasoning

In NEXO, the case of the day is fictional, with a real clinical presentation and up to six progressive clues, and you propose the diagnosis by searching for the ICD-10 code. Noticing that contractions in a pregnant woman may be related to the week's heat requires the same exercise: weighing each new clue and revising the hypothesis, as in training differential diagnosis with clues.

The limits of using the game are described on the NEXO economy page. If you research the teaching of clinical reasoning, the page for researchers explains how to get in touch. Professionals who write or review cases find information under curation and under creators, and institutions can learn about the proposal on the partnerships page. Searching for the right ICD-10 code completes this training.

Frequently asked questions

Can heat cause preterm birth?

The reviews show a consistent association: the chance of preterm birth rises by about 5 percent for each additional degree and by 16 percent during heatwaves (Chersich et al., 2020). The association is population-level and does not allow predicting the outcome of a particular pregnant woman.

At what stage of pregnancy does heat weigh most?

In high-income countries, the effect is concentrated in the last weeks, a few days after exposure (Chersich et al., 2020). In low- and middle-income countries, the risk also appears in the first trimester (Raghavan et al., 2025).

From what temperature does the risk increase?

A meta-analysis estimated a 5 percent increase in risk from a maximum temperature of about 30.5 °C (Heil et al., 2026, Nat Commun). The thresholds still vary according to the index and the location, and the region's official alerts are the practical reference.

Does a fan help a pregnant woman in the heat?

It helps when the temperature is below 40 °C, according to the WHO (WHO, fact sheet). Above that, the WHO advises other means of cooling, such as cool showers and a cool place for a few hours.

How much water should a pregnant woman drink on hot days?

The WHO suggests for the general public about one glass of water per hour and at least two to three liters per day during heatwaves (WHO, fact sheet). Pregnant women with fluid restriction due to another disease need individual guidance.

Take-home messages

• Treat extreme heat as an obstetric risk factor, linked to preterm birth, stillbirth and low birth weight (WHO, UNICEF and UNFPA, 2023).
• Remember that the risk grows with the intensity and duration of the heatwave (Chersich et al., 2020).
• Pay special attention to the third trimester and the days right after a heat wave (Fitch et al., 2026).
• Ask about work, home, water and cooking, because exposure is unequal (WHO, fact sheet).
• Use WHO guidance on shade, ventilation, water and official alerts as the basis for counseling (WHO, fact sheet).
• Include heat guidance in the prenatal and postpartum routine (Lusambili et al., 2026).

References

1. World Health Organization, UNICEF, UNFPA. Protecting maternal, newborn and child health from the impacts of climate change: call for action. Geneva: WHO; 2023. Publication page.
2. World Health Organization. Climate change, heat and health. Fact sheet. Full text.
3. Chersich MF, Pham MD, Areal A, et al. Associations between high temperatures in pregnancy and risk of preterm birth, low birth weight, and stillbirths: systematic review and meta-analysis. BMJ. 2020;371:m3811. PMID 33148618. DOI 10.1136/bmj.m3811.
4. Lakhani S, Ambreen S, Padhani ZA, et al. Impact of ambient heat exposure on pregnancy outcomes in low- and middle-income countries: a systematic review. Womens Health (Lond). 2024;20:17455057241291271. PMID 39500870. DOI 10.1177/17455057241291271.
5. Raghavan S, Isaac TS, Arya D, et al. Association between heat and air pollution (PM2.5 and black carbon) exposure in pregnancy and preterm birth in low- and middle-income countries: a systematic review and meta-analysis. BMJ Paediatr Open. 2025;9(1):e003428. PMID 40953917. DOI 10.1136/bmjpo-2025-003428.
6. Heil CS, Ioannou LG, Alce G, et al. A systematic review and meta-analysis on heat thresholds for maternal health and birth outcomes. Nat Commun. 2026;17(1):8768. PMID 42624863. DOI 10.1038/s41467-026-76822-8.
7. Li Z, Li S, Su S, et al. Extreme temperature events exposure and risks of stillbirths: nationwide large birth cohort study in China. Environ Int. 2025;204:109854. PMID 41101031. DOI 10.1016/j.envint.2025.109854.
8. Fitch A, Huang M, Strickland M, et al. Preterm and early term delivery after heat waves in eight US states: a case-crossover study using the High-Resolution Urban Meteorology for Impacts Data Set (HUMID). Environ Health Perspect. 2026;134(2):184-194. Full text in PMC.
9. Holvoet E, Lokubal P, Chersich M, et al. The impact of ambient heat on perinatal and neonatal health in the context of climate change: a scoping review. Curr Environ Health Rep. 2026;13(1):29. PMID 42429868. DOI 10.1007/s40572-026-00549-3.
10. Brimicombe C, Conway F, Portela A, et al. A scoping review on heat indices used to measure the effects of heat on maternal and perinatal health. BMJ Public Health. 2024;2(1):e000308. PMID 40018117. DOI 10.1136/bmjph-2023-000308.
11. Persson I, Sturm A, Manchikanti S, et al. Impacts of climate change during pregnancy, perinatal period and infancy: a systematic review. BMJ Paediatr Open. 2025;9(1). Full text in PMC.
12. Álvarez-García C, Aviles-Gonzalez CI, Sanz-Martos S, et al. The impact of climate change on pregnant women's health: a scoping review of reviews. Nurs Rep. 2026;16(9):332. PMID 42784097. DOI 10.3390/nursrep16090332.
13. Lusambili A, Scorgie F, Chersich M, et al. Left to cope alone: how pregnant and postpartum women and their newborns manage extreme heat in coastal Kilifi, Kenya. Front Glob Womens Health. 2026;7:1825348. PMID 42936054. DOI 10.3389/fgwh.2026.1825348.
14. Heil CS, Sawry S, Frennert S, et al. Mobile heat early warning system for pregnant and postpartum women in South Africa: prospective observational evaluation study. JMIR Mhealth Uhealth. 2026;14:e102863. PMID 42790880. DOI 10.2196/102863.