Darshnika Lakhoo spent years studying how rising temperatures affect pregnancy, documenting evidence that heat can increase the risk of premature births, pregnancy complications and infant deaths. But as she continued her research, the medical scientist found herself confronting the same danger she had been warning others about.

In late 2024, as Johannesburg endured repeated heat waves, Lakhoo was pregnant with twins and experiencing several serious complications. On Nov. 5, her research team published a study in the medical journal Nature Medicine examining the effects of rising temperatures on pregnancy. Just days later, Lakhoo went into labor six weeks before her due date.

"No one is going to be safe in a warming world," said Lakhoo, now 35. "And especially not pregnant women."

A growing body of research from different parts of the world is revealing how rising temperatures threaten pregnant women and their unborn children, increasing the likelihood of miscarriages, stillbirths, congenital abnormalities, high blood pressure and premature births.

The danger extends well beyond countries accustomed to extreme heat. Studies indicate that pregnancy risks increase as temperatures rise above the conditions women normally experience, meaning those living in temperate and cooler climates are also vulnerable.

Yet many health professionals are only beginning to recognize heat as a significant pregnancy risk, often focusing on extreme temperatures rather than the cumulative effects of unusually hot weather.

The consequences could be substantial as climate change makes heat waves more frequent, longer-lasting and geographically widespread.

A Reuters investigation found that heat waves have evolved over the past 65 years from relatively limited weather events into enormous systems capable of covering much of the planet. The investigation uses the term "heat storm" for the largest events, defined as heat waves that, at their peak, cover at least 1% of the Earth's surface.

Under the Reuters definition, a heat wave consists of five consecutive days when temperatures rank among the hottest 10% recorded historically for a particular area.

There were just 50 heat storms during the 1960s. The largest covered about 6% of the planet. This decade, more than 665 have already occurred, with the biggest lasting a year and spreading across nearly 85% of the Earth's surface.

For pregnant women, the consequences are increasingly difficult to ignore.

An original Reuters analysis of millions of birth records in Mexico found that heat storms and ordinary heat waves during the final month of pregnancy were associated with an increase of up to 18% in premature births.

Across the country, unusually high temperatures during the four weeks before delivery corresponded with a 6.5% increase in the rate of premature births, equivalent to more than 1,400 additional cases annually during hot seasons.

The findings add to mounting evidence that even relatively modest temperature increases can affect the health of mothers and babies.

The growing medical risks of pregnancy

Lakhoo's interest in the relationship between climate and health developed during her early career as a doctor working with patients in Johannesburg's densely populated townships.

She later became a research clinician affiliated with the University of the Witwatersrand, where she joined a team examining the effects of rising temperatures on maternal health.

By the time she and her partner, Mark Robinson, decided to have a baby, Lakhoo was already investigating the risks that heat posed to pregnant women.

Her research team, led by Matthew Chersich, a maternal health and climate change specialist at the university, reviewed nearly 200 scientific studies to understand how rising temperatures affect pregnancy outcomes.

The findings showed that the risks increase with temperature, although researchers have not established a single threshold that applies everywhere.

One major study identified 30 degrees Celsius (86 degrees Fahrenheit) as a point at which pregnancy complications begin rising significantly. Research involving 14 lower-income countries, however, found that risks associated with premature births and stillbirths can emerge at even lower temperatures.

Lakhoo and her colleagues found that a woman's exposure to temperatures above the normal conditions in her area matters more than a universal temperature threshold.

For every 1 degree Celsius increase in average local temperatures, the risk of stillbirth in the following weeks rises significantly, their review found.

The studies also linked higher temperatures to increased risks of miscarriages, congenital abnormalities and placental abruption, a potentially life-threatening condition in which the placenta separates from the uterus before delivery.

Heat can also worsen maternal health conditions, particularly high blood pressure and preeclampsia.

Preeclampsia is a pregnancy complication characterized by high blood pressure that can damage the kidneys and other organs. Severe cases can cause kidney failure, heart attacks and strokes. The condition accounts for more than one in seven maternal deaths worldwide.

A study tracking 2 million women in China found that unusually high temperatures were associated with a 13% increase in the likelihood of developing preeclampsia.

Women with healthy pregnancies are not immune. However, heat can compound existing biological and social vulnerabilities, increasing the danger for women with chronic illnesses, older mothers, smokers and those taking prescription medication.

Twin pregnancies are particularly vulnerable, Chersich said.

One of the strongest findings in Lakhoo's review concerned premature births, which can expose children to lifelong health complications, including impaired growth, severe asthma, intellectual disabilities and cerebral palsy.

Analyzing studies from four continents, her team found that every additional 1 degree Celsius in average temperatures was associated with a 4% increase in the odds of premature birth in the following days.

Following a heat wave, the odds increased by 26% during the subsequent week.

Other research has documented similar patterns. In New York, warm-season heat waves have been associated with more emergency department visits by pregnant women experiencing miscarriages, diabetes and high blood pressure. Researchers have also linked the events to an increase in babies born with cataracts that can cause blindness.

Studies in Brazil have found more hospitalizations among pregnant women during heat waves, while research in India has linked high temperatures to an increase in babies requiring intensive care.

Reuters examined more than 8 million birth records in Mexico between 2008 and 2025 to investigate the relationship between heat exposure and premature births.

The database included births from regions with dramatically different climates, from hot deserts and humid jungles to cooler mountain plateaus. Although it did not provide detailed information about mothers' pre-existing medical conditions or socioeconomic circumstances, it allowed researchers to compare birth outcomes with temperatures across different regions.

The analysis found that heat storms and ordinary heat waves during the four weeks before delivery were associated with a roughly 6.5% increase in premature births.

The effect became more pronounced as exposure intensified. Mild heat exposure corresponded with an increase of about 4%, while the highest exposure was associated with an 18% increase.

The findings suggest that the number of hot days and the intensity of exposure both matter.

A pregnancy under extreme heat

Lakhoo discovered she was expecting twins just weeks after she and Robinson began trying for a baby.

During an ultrasound appointment, her gynecologist noticed something unusual and requested an additional scan. Lakhoo recalled feeling her legs shake as she waited for the results.

The scan revealed two babies.

Although she was delighted, the news brought additional concerns. Twin pregnancies carry a higher likelihood of premature delivery because of the physical demands of carrying two babies.

At the same time, southern Africa was experiencing an exceptionally hot period. The year 2024 was on track to become the hottest on record globally, while parts of the region endured severe drought and water shortages.

Johannesburg, situated more than a mile above sea level, is generally less exposed to extreme temperatures than many other parts of South Africa. But the city experienced repeated heat waves, prompting authorities to warn residents against outdoor activities because of the risk of heat exhaustion.

The drought devastated crops across southern Africa, contributing to what was described as the region's worst drought in a century. Some schools closed classrooms after inspectors warned of the dangers of heat stroke, exhaustion and potentially fatal heat-related illnesses.

South Africa's vice president also collapsed from the heat during a speech.

A Reuters analysis found that Johannesburg experienced 74 heat-storm days during Lakhoo's pregnancy, more than a quarter of the length of a full-term pregnancy.

Temperature records from monitoring stations near her home showed that she experienced approximately 30 days above 30 degrees Celsius, with more than half occurring during the final month of her pregnancy.

The heat made everyday activities increasingly difficult. Lakhoo struggled to sleep, sometimes pushing Robinson out of bed in an attempt to find relief from the rising temperatures. At times, she recalled, even breathing felt exhausting.

Chersich understood the danger. Lakhoo was researching the very complications she was beginning to experience.

Concerned about her exposure, he sent her a portable air conditioner. Although research into the most effective ways to protect pregnant women from heat remains limited, air conditioning is widely regarded as an important protective measure.

Chersich hoped it would help Lakhoo keep the babies in the womb for another week or longer.

"Air conditioning is lifesaving equipment," he said. "I couldn't understand why she didn't get it herself."

Lakhoo, however, used the device sparingly. She had never previously owned an air conditioner and considered the electricity costs too high on her modest research salary.

She also struggled to see herself as particularly vulnerable, given that much of her research focused on poorer women with limited access to cooling facilities.

Her attention remained largely on the additional risks associated with carrying twins.

As the pregnancy progressed, medical complications began to emerge.

At around three months, her obstetrician identified abnormalities in the blood flow to her placenta, according to Lakhoo and medical records she shared with Reuters.

By eight months, her blood pressure had increased, requiring prescription medication. A week later, tests detected protein in her urine, another warning sign of preeclampsia.

Each development reflected conditions that research has associated with higher temperatures.

In early November, after several days of unseasonably hot weather in Johannesburg, doctors discovered that the twins had stopped growing, a complication that can occur in multiple pregnancies.

They scheduled an early cesarean section. But before the procedure could take place, Lakhoo's water broke and labor began.

Robinson rushed her to the hospital, where doctors delivered the babies.

He remembered their skin appearing blue and gray, describing their tiny bodies as resembling "frozen supermarket chickens."

The twins were so premature that they struggled to breathe and immediately required intensive medical care.

While nurses rushed the newborns to the neonatal intensive care unit, Lakhoo was taken to the maternity ward. She recalled an empty bassinet in one corner of her room and the sound of other newborns crying in the hallway.

It is impossible to determine whether heat directly caused Lakhoo's complications or brought forward her delivery. Carrying twins had already placed her pregnancy at elevated risk.

But she believes the extreme temperatures made an already difficult situation worse.

"The heat brought that date closer," she said.

Doctors notice a pattern in Mexico

The heat storm that affected Johannesburg during Lakhoo's pregnancy also spread across Mexico City, where it persisted for much of the spring and summer of 2024.

The Mexican capital, located 2,240 meters (7,350 feet) above sea level, is usually protected from the most extreme temperatures by its elevation.

But the heat shattered records, reaching 34.7 degrees Celsius in May.

At Juarez Hospital, a 179-year-old medical institution in Mexico City, Dr. Juan Rivero Corona began noticing more pregnant patients arriving with preeclampsia and other high blood pressure disorders.

Rivero Corona, who leads the hospital's gynecology and obstetrics unit, also observed an increase in placental abruptions, a medical emergency that can cause severe maternal bleeding and a sudden drop in the fetus' heart rate.

In serious cases, doctors have only minutes to perform an emergency cesarean section to save the lives of both mother and child.

Initially, Rivero Corona considered whether other factors, including obesity, diabetes and maternal age, could explain the increase.

But he began questioning whether extreme heat was also contributing.

Some pregnant women arrived at the hospital flushed and struggling to catch their breath after traveling on hot buses. Their blood pressure was sometimes so elevated that nurses asked them to sit and cool down before checking their vital signs.

Rivero Corona began advising patients to drink more water and wear loose clothing. The hospital also moved appointments for pregnant women to cooler early-morning hours.

When he reviewed hospital records, he found that cases of high blood pressure disorders, placental abruptions and premature births consistently increased during spring, the hottest season in Mexico City.

The Reuters analysis also found evidence of the relationship in the southern Mexican state of Chiapas, where temperatures are generally cooler because of the mountainous terrain.

When heat storms or heat waves occurred during the final month of pregnancy, premature birth rates increased by approximately 9.6%.

The findings were particularly concerning for women working outdoors.

Linda Lopez, 28, who lives in Chiapas, said intense hot flashes became a regular part of her pregnancy as summer rains arrived late and a heat storm settled over the region.

Temperatures reached 28.5 degrees Celsius, prompting her to stop working in the coffee fields alongside her husband.

Lopez, who is due in January, regularly visits a local midwife, Amalia Sanchez, who massages her stomach to relieve discomfort and prepares tea from white maravilla flowers to treat what she diagnosed as a urinary tract infection, a condition often associated with dehydration.

By the time she spoke to Reuters, Lopez had spent more than 80% of her pregnancy during a heat storm.

Sanchez, who has delivered babies for more than 50 years, said the consequences of heat extend beyond the mother.

"Everything the mother lives through, the baby suffers," she said.

Scientists search for answers

Despite mounting evidence, researchers still have significant gaps in their understanding of how heat affects pregnancy.

The impact varies considerably between regions. Reuters found a strong relationship between heat exposure and premature births across Mexico, but some states, including Baja California, recorded little change.

Scientists say several factors could explain the differences, including age, body size, humidity, wind conditions, housing quality and access to air conditioning.

The timing of exposure also appears to matter, although researchers have not established a consistent pattern for every pregnancy complication.

Heat exposure early in pregnancy has been associated with birth defects and high blood pressure, while rising temperatures during the final week before delivery have been linked to stillbirths.

Evidence concerning premature births is more complicated. Some studies identify the weeks immediately before delivery as particularly dangerous, while others suggest there may be several vulnerable periods throughout pregnancy.

Scientists are also investigating the biological mechanisms behind the effects.

One theory is that high temperatures force a pregnant woman's body to redirect blood away from the placenta to help regulate its own temperature, potentially depriving the fetus of oxygen and nutrients.

Others believe heat increases the strain on a fetus' heart, disrupts the release of essential hormones or interferes with genes responsible for the development of fetal organs.

Researchers at Rahima Moosa Hospital in Johannesburg are trying to establish more precise answers through an extensive study of pregnant women and their newborns.

The hospital sits between Johannesburg's affluent suburbs and its densely populated townships, serving women from communities with widely differing living conditions.

Under a research project led by Dr. Renate Strehlau, pregnant women are recruited through local clinics and monitored throughout their pregnancies.

Participants wear devices that measure their daily heat exposure and regularly visit the hospital for medical assessments before and after delivery.

Researchers collect information about their living conditions and conduct tests on their blood, urine and other biological samples. Some women also donate breast milk, placentas and umbilical cords.

The team examines the samples for signs of inflammation, elevated hormone levels, kidney problems and metabolic changes that could help explain how heat affects mothers and developing babies.

For women participating in the study, the risks are often visible in their everyday experiences.

Nkazimulo Nkiwane, 30, recalled suffering persistent headaches and sleepless nights as temperatures climbed during her pregnancy.

She rented a single room near the home where she worked as a cleaner. With limited access to cooling facilities, she often sat outside her cramped accommodation hoping to catch a breeze.

At times, she said, the heat became so overwhelming that she cried.

Like many women involved in the research, Nkiwane had not known that high temperatures could harm her pregnancy.

As the months passed, nurses noticed her blood pressure rising. She eventually underwent an early cesarean section.

"There was too much pain," she said of the procedure. Her baby survived.

Strehlau compared the research into heat and pregnancy to the early years of the HIV epidemic, when medical understanding lagged behind the realities facing patients.

"The science was definitely behind reality, and I think it's similar to what we're seeing with heat," she said.

She believes greater awareness could help prevent some of the worst outcomes, but says maternity care must change.

Medical professionals should routinely ask pregnant women about their exposure to high temperatures, just as they inquire about smoking and alcohol consumption.

Rising blood pressure and other possible signs of heat-related complications should prompt further assessment and medical attention.

Lakhoo argues that protecting mothers also requires broader changes to workplace and public health policies.

Pregnant women should have access to maternity leave before delivery, she said, rather than being expected to continue working in dangerous temperatures until shortly before giving birth.

She also advocates freely available drinking water and mobile cooling facilities to help women who cannot afford air conditioning.

The challenge is particularly significant in South Africa, where only about 7.5% of households own an air conditioner, according to government figures.

Millions of people also depend on crowded minibuses for their daily journeys. Recent research has found that temperatures inside these vehicles can reach 39 degrees Celsius (102 degrees Fahrenheit).

For women who spend hours commuting or working outdoors, avoiding heat exposure can be nearly impossible.

Protecting mothers from the heat

Miami, one of the United States' most heat-exposed major cities, offers another example of the growing inequality in access to protection.

A Reuters analysis found that the city experienced 16 heat-storm days during the 1960s. This decade, it has already recorded more than 1,000.

The growing threat has prompted local medical professionals to develop initiatives aimed at protecting pregnant women.

Clinician Cheryl Holder began working with a nonprofit organization to raise awareness and provide practical assistance to expectant mothers.

At an early meeting, Holder explained the medical risks to a group of about 20 birth doulas, who responded with accounts of similar experiences among their clients.

"They were seeing it in their clients," Holder recalled.

Her team now warns pregnant women about the dangers of heat and distributes basic supplies, including water bottles, towels that can be soaked and wrapped around the head or neck, umbrellas and electric fans.

Some women also receive air conditioning units.

The initiative primarily targets lower-income mothers, who often have fewer options for escaping high temperatures.

More affluent women, meanwhile, are increasingly taking precautions of their own.

Valentina Patruno, 44, who lives in western Miami, said she was surprised to discover she was expecting another child.

"It was sent by God," she said.

Her doula warned her about the potential effects of extreme heat, prompting her to dramatically change her daily routine.

As temperatures climbed during the summer, Patruno increasingly avoided outdoor activities, including walks in a nearby bird sanctuary and visits to Miami's beaches.

She limited beach visits to nighttime and only ventured out for walks when cloudy weather offered some relief.

Even then, she said, she quickly felt the heat affecting her body and returned indoors.

Her husband, Luigi Gutierrez, said the city's summer conditions made it difficult to spend time outside.

"Here in Miami, we only have two seasons: nice, and Hell," he said.

Patruno described her pregnancy as "my air-conditioned life."

Hours before Reuters published its report, she delivered her baby, one day before her due date.

Living with the consequences

Lakhoo's twins spent weeks in intensive care before they became strong enough to breathe reliably without assistance.

After giving birth, she was so desperate to see them that she asked nurses to remove her catheter so she could visit the neonatal unit.

When doctors finally allowed her to hold the babies, she sat between their incubators as a nurse disconnected the tubes and placed them against her chest, covering them with a blanket decorated with hearts.

It was, she said, "my happiest moment."

During one visit, a nurse mentioned that the neonatal intensive care unit was unusually full. When Lakhoo asked why, the nurse described a scientific article circulating among hospital staff that linked heat waves to premature births.

Lakhoo realized she was discussing the research she and her colleagues had published.

The moment brought a complicated mix of emotions. Her work was finally reaching medical professionals, but it remained inseparable from the traumatic experience of her own pregnancy.

Nearly two years later, the experience continues to shape how she approaches heat.

She no longer hesitates to use air conditioning, despite the additional electricity costs. With two children who were born prematurely, she considers it a necessity rather than a luxury.

"Everyone is at risk," she said.

In August, during Johannesburg's winter, Lakhoo and Robinson took their daughters to a favorite park.

Robinson carried one child under each arm before setting them down on the grass. The girls, sporting untidy ponytails and pigtails, immediately began chasing each other, laughing as they fought over a discarded red bottle cap.

Lakhoo followed them while Robinson stood nearby catching his breath.

The twins have grown into energetic toddlers, although their premature birth remains visible in their development. Lakhoo said they are shorter than 98% of children their age.

Their skin also carries small scars from the tubes and intravenous lines that sustained them during their first weeks in intensive care.