When the Weather Meets a Pregnancy: What Hot and Cold Seasons May Mean for Early Babies

Four 2026 studies from the United States, Canada and worldwide reviews on temperature, air quality and premature birth

Researchers in 2026 asked whether very hot or very cold weather during pregnancy is linked to babies arriving early, and whether a mother's surroundings relate to how extremely premature babies do in intensive care. Both extreme cold and extreme heat were linked to earlier birth, and cold, little greenery and high ozone together were linked to fewer tiny babies surviving without major complications. These are patterns, not proof of cause, and most pregnancies end well in any weather.

Many parents of babies in the neonatal intensive care unit (NICU) wonder what they could have done differently. The honest answer, most of the time, is that early birth has many causes and it is almost never anyone's fault. Still, scientists keep studying the world around pregnant women, because the climate is changing and weather is something communities can prepare for together. Four recent studies shed new light on this question, and this article explains what they found and what they do not show.

How the question grew over the years

Researchers have long wondered whether the weather plays a part in early birth. For families and doctors, that left little concrete to say beyond general advice, so over the past decade researchers began to check the question carefully. Early studies looked at high temperatures and early delivery in certain regions [1], and later ones examined enormous collections of records, including 32 million births in the United States [2]. Researchers then combined the results of many studies. One such combined analysis of 47 studies [3] found, as described by the 2026 US researchers, that the chance of a premature birth rose about 5% for each extra degree Celsius and 16% during heatwaves. A larger 2025 review [4] widened the view to include mothers, unborn babies and newborns.

But important questions were left open. Cold weather had received less attention. Little was known about what happens after a very premature baby is admitted to the NICU, and whether the mother's surroundings matter then. Researchers also could not say at what temperature the risk begins, which health authorities need if they are to issue warnings. Some earlier work had found links between temperature extremes and NICU admission [5], and between a mother's temperature exposure and a baby's lung function [6], but the picture was incomplete.

The four 2026 studies each add a piece. One US study asks whether hot and cold spells change when babies are born [7]. A Canadian study asks how extremely premature babies fare, depending on their mother's environment [8]. Two reviews, which are summaries of many earlier studies, look at heat and newborn health more broadly [9] and try to pinpoint the temperature at which risk begins to rise [10].

How the researchers went about it

The US team [7] used hospital records from more than 200,000 pregnancies (201,759 single-baby pregnancies) at 19 hospitals in 12 states between 2002 and 2008, collected by a research group called the Consortium on Safe Labor. They counted how many days of a pregnancy were unusually hot or unusually cold for that hospital's area, and compared women with many such days against women with a typical mix of weather.

The Canadian team [8] looked at 14,748 babies born between 22 and 28 weeks of pregnancy (full term is about 40 weeks) who were cared for in the country's 32 main NICUs between 2010 and 2020, through a group called the Canadian Neonatal Network. Using the mother's postal code, they estimated how green the area was, how much air pollution it had and how warm or cold it was. They then looked at one main result: whether the baby survived without any major complication, such as serious bleeding in the brain or long-term lung problems. About one in three babies (32.1%) did.

The two review teams read and combined published research rather than studying families directly: one mapped 61 studies [9] and the other gathered 103 studies, using 21 of them to calculate temperature thresholds [10]. Both belong to a European research project on heat and health.

What they found

Early birth and temperature. In the US study [7], women who lived through many extremely cold days had 1.66 times the odds of delivering before 34 weeks compared with women with moderate weather, and women with many extremely hot days had 1.20 times the odds. "Odds" is a way of comparing chances; it does not mean that most of these women delivered early. Across the whole group, about 3 in 100 babies were born before 34 weeks. Cold seemed to matter most in the first and last parts of pregnancy, and heat in the middle part. The researchers stress that they cannot show that weather caused any birth.

Babies already born early. In the Canadian study [8], hot weather made no noticeable difference. What stood out was the surroundings. Mothers living in areas with high levels of an air pollutant called ozone had babies somewhat less likely to survive without a major complication, and the pattern was stronger when ozone, less greenery (fewer trees, parks and plants near home) and the coldest third of Canadian climates, with long and very cold winters, came together. For example, among 2,648 babies whose mothers had none of these three exposures, 35.0% survived without major complications. Among 240 babies whose mothers had all three, 24.6% did. Please keep this in perspective: these are averages for large groups drawn from where families live, so they say little about any one baby. The authors also note that some of the ozone link may reflect city living, closer to advanced NICU care, and that finding any link in a wealthy country such as Canada is a reminder to keep looking.

Heat for newborns and the wider world. The first review [9] found that nearly all studies linked heat with problems such as low birth weight, stillbirth, newborn death and birth defects, and that families with fewer resources were affected most. Almost no studies had measured the heat that babies themselves are exposed to after birth, and babies in NICU incubators are kept in carefully controlled warmth. The second review [10] estimated that, for pregnancy and birth problems as a whole, risk starts to rise once daytime highs go above about 24.5 °C, and is about 5% higher at 30.55 °C, 10% higher at 34.11 °C and 20% higher at 39.07 °C. These are modest shifts in risk across whole populations, not predictions for any one pregnancy.

Why the studies look different but fit together

It may seem odd that heat showed up in the US study and in the reviews, but not in the Canadian one. The studies measured different things. The US team counted unusually hot days for each place, the Canadian team used nine-month averages for areas where summers are mild, and its warmest third had typical monthly highs of about 24 °C. The Canadian authors also suggest that air conditioning, which they note is common in Canada, may soften the effects of heat. The reviews were built from many places around the world and many ways of measuring heat. Each study gives one piece of a larger puzzle.

What this might mean for families

First, these findings are not a reason to feel guilt or fear. Weather is only one of many influences on pregnancy, and the studies describe groups of people, not what will happen to you. Second, simple everyday care during extreme weather is sensible: staying cool, drinking enough fluids and avoiding strenuous outdoor activity in heat, and keeping warm in severe cold. These are common-sense steps. The studies did not test whether they prevent early birth. Third, families who face energy poverty, outdoor work or unsafe housing deserve extra support, because the reviews found that disadvantage made heat harms worse. If you are pregnant and worried about the weather, your midwife or doctor can tailor advice to you.

For parents who already have a baby in the NICU, remember that the unit manages your baby's temperature carefully, whether it is a hot day or a cold one, and that the Canadian findings describe patterns among thousands of babies and cannot be applied to an individual child. If you would like to learn about how NICU teams protect babies from the cold, see our article on the neonatal hypothermia bundle.

What researchers are working on next

Researchers want to repeat these studies with newer information and from more places, including hotter, lower-income regions where very little is known. A study across hospitals in sub-Saharan Africa [11] has linked extreme heat with stillbirth and newborn deaths, one estimate [12] suggests warming is behind a share of newborn deaths in poorer countries, and community interviews in Kenya [13] describe how hot weather can distress newborns and make care harder. Scientists are also studying how heat acts on the body of a pregnant woman and her baby [14], [15], and health experts continue to track the wider health dangers of extreme heat [16]. Researchers are also hoping to follow children into later years to see whether early exposures matter for growth, breathing and learning. Another goal is better warning systems that give pregnant women and newborn units timely, practical advice. For related reading on how survival differs around the world, see our article on global inequities in extremely preterm survival.

A hopeful closing thought

The four studies agree on one broad idea: both very hot and very cold weather are linked with earlier birth, and some neighbourhood conditions are linked with how tiny babies do. They do not prove cause, and no one can read an individual family's risk from them. What they offer is a reason for communities and hospitals to prepare, protect and support families, especially when extreme weather arrives.

References

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