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EXTREME HEAT CAN PUSH PREGNANT WOMEN INTO PRETERM LABOUR
The increased frequency and intensity of heatwaves put pregnant women at a higher risk of preterm and early-term labor, stillbirth, preeclampsia, and high blood pressure
By Sigrid Vestergaard Frandsen, Director of Environmental Health
08/26/2026
She felt it on the third night of the heatwave — a tightening low in her belly, sharp enough to make her stop what she was doing and grip the counter. She was thirty-one weeks along. She told herself it was nothing. The fan in the bedroom had stopped working two nights ago, and she hadn't slept properly since. Outside, it was on its fourth consecutive day above 38°C, and inside her apartment, with no air conditioning and windows that only opened a few inches, it wasn't much cooler.
She didn't know that her body was already responding to the heat the way every pregnant body does under enough strain: pulling blood away from her placenta and sending it to her skin, trying to cool itself the only way it knew how. She didn't know that the dehydration she'd been fighting off with lukewarm tap water all week was quietly reducing blood flow to her uterus. She just knew the tightening kept coming back, closer together each time, and that by morning, she was in an ambulance, nine weeks earlier than her due date.
This description captures the experience of thousands of women, particularly after the heatwaves of 2026, in cities that never cool at night, and where people can't afford air conditioning or the energy to run it.
Heat and preterm births
The results vary by country or state, but they all show a very clear connection between heat and birth complications: the largest study on this topic pooled 70 cases across 27 countries. It found that the odds of preterm birth rose by about 5% for every 1°C of warming, and by 16% during heatwaves. A California study looking at nearly 60,000 warm-season births showed preterm birth rose 8.6% for every 10°F rise in the week before birth. A nationwide study looking at 223,000 deliveries at 12 different US sites showed a 5°F rise in the final week of pregnancy raised the risk of early delivery by 12 to 16%. A Canadian study showed that four to seven straight days at or above 32°C raised the chance of an early term birth by 27% in Montreal.
Heat and stillbirths
One study looking at eight stillbirths pointed the same way. Stillbirths rose about 5% for every 1°C, and were more than three times higher when the hottest exposures across a trimester or a whole pregnancy were compared with the coolest. Another stillbirth study across 14 lower- and middle-income countries found hot days in the week before birth raised the risk of both preterm birth and stillbirth, with risk climbing significantly when temperatures reached above ~20°C (68ºF).
Heat and birth weight
Extreme heat events have been linked to a drop in average birth weight of roughly 55 to 97 grams, and to placentas that are smaller and less efficient. When a pregnant woman overheats, her body sends blood toward the skin to cool itself, and away from the placenta that feeds her baby. Another study found that dehydration reduces blood flow to the uterus. Heat stress can tighten the uterus and trigger labor before the baby is ready.
Heat and maternal illnesses
Other research links extreme heat to a doubling of severe maternal illness, including cardiac and cerebrovascular events, and to a rise of up to 20% in the odds of gestational diabetes. Preeclampsia, a dangerous rise in blood pressure that can progress to seizures, appears more often among women who conceive in the warmest months. In South Asia, heat in the third trimester was tied to higher rates of hypertension and severe preeclampsia.
One study found that blood volume rises by close to half in a pregnant body that is already running hot. The growing fetus and placenta produce heat of their own. Added heat means added strain. There is simply less room left to shed heat, so temperatures that merely bother most adults can push a pregnant woman past her limit. Yet reviews of European heatwave plans found most did not name pregnant women as a high-risk group. Allowing heat to work unopposed.
Socio-economic factors
Lastly, socio-economic factors play a crucial role, too. The harm falls hardest on women with the least income, the least education, and the least access to cooling. In lower- and middle-income countries, the risk runs through the whole pregnancy rather than only the final weeks. Neighborhoods without tree cover or air conditioning can be up to 7°C hotter than greener streets nearby. Women who farm, trade, or work outdoors carry that heat all day.
A 2024 study of birth outcomes in Massachusetts found that living in an environmental justice community, one facing compounded exposure to pollution and climate hazards, was linked to a higher risk of preterm birth and low birthweight across nearly every ethnic group. But the disparity ran deeper than geography: Black women and Native American and Alaska Native women faced double the risk of preterm birth compared with white women, a gap that held even outside those high-exposure communities.
Heat does not look like a disaster. There is no wind howling, no wall of water tearing down houses. A heatwave arrives quietly, settles over a city for a few days, and leaves just as quietly. By the time it's gone, the number of women going into preterm labour will have risen 27% compared to a period with normal temperatures — and almost no one will have noticed it happening. It is an invisible, dangerous cycle playing out behind closed hospital doors, far from any headline. But we now have enough evidence to be able to call climate change a threat to pregnant women and their babies.