How Extreme Heat May Be Raising the Risk of Gestational Diabetes

Last year, when Pooja Patil went for a monthly blood test during the second trimester of her pregnancy, she was taken aback by the reports.

She had developed gestational diabetes mellitus (GDM), a condition in which hormones produced by the placenta make the body less responsive to insulin, which leads to an inability to control blood sugar levels during pregnancy. When GDM goes unmanaged, it can increase the risk of complications such as preterm birth, preeclampsia, and stillbirth.¹

GDM can be difficult to recognize because many of its signs, such as tiredness or frequent urination, are also common during pregnancy. Women may also experience unusual thirst or recurrent infections, especially of the urinary tract or vagina. While the exact reasons some women develop GDM are not fully understood, common risk factors include obesity, physical inactivity, and a family history of diabetes, among others.

32-year-old Patil had none of the classical risk factors. She stayed active, ate well, and had been mindful of her health, but she still developed the condition. “At first, I couldn’t believe it,” she said.

After being prescribed medication for GDM, her health still worsened. She often felt dizzy and had little strength to even get out of bed. “Sometimes it got unbearable, and I felt anything could happen to me or my baby,” she said.

Struggling to make sense of the diagnosis, she turned to her doctor and community healthcare workers in Shirol, a town in western India’s Maharashtra state, asking what might have caused it. In her conversations with over 50 women, she found that a majority of them suffered from heat stress during their pregnancy. After hearing this, she realized that at the end of her first trimester, she too had endured severe heat. Later, the doctor told her that heat may have played a role in worsening her condition.

Stories like Patil’s are becoming common. GDM now affects 14 percent of pregnancies globally.² In India, researchers found that nearly one in four pregnant women has GDM, with over five million women affected by it.^3,4

A growing body of research shows that climate change-driven extreme heat may be increasing the risk of GDM. Studies from around the world are also pointing to critical windows of vulnerability, suggesting that rising temperatures may be shaping maternal health in overlooked ways. Emerging evidence suggests GDM may be shaped not just by biology, but by the environment too.

“GDM is usually discussed in terms of individual risk factors, such as living with obesity or family history, while other factors, like heat, receive less attention,” said Anais Teyton, a postdoctoral researcher in environmental epidemiology at the Institute for Environmental and Occupational Health.

Pregnancy Under Relentless Heat

From the time she was diagnosed with GDM, Patil was deeply anxious. “My condition was so severe that the doctor had warned of possible harm to the baby,” she said.

At the same time, temperatures in her village were climbing past 40°C (104°F), part of a pattern of increasingly intense and prolonged heat in recent years. There was little relief from the heat, even at night. A fan only pushed around hot air, while rising humidity disrupted her sleep.

Doctors advised her to stay hydrated and avoid stepping out during periods of peak heat, but the nights were the hardest. “I couldn’t sleep most nights and would sweat the entire time.” What she expected would pass in a week or two stretched on for three months as the heat persisted. “I never imagined it would get so hot in this region,” she added, as indoor temperatures stayed above 28°C even at night. When she gave birth, she felt a deep sense of relief. “I survived by a stroke of luck.”

Recent studies suggest that such prolonged heat exposure during pregnancy can carry an increased risk of developing GDM. Scientists led by Laura Granés, a postdoctoral researcher at the Primary Health Care University Research Institute Jordi Gol, found that women exposed to sustained high temperatures over one month had about twice the risk of developing GDM.⁵ Analyzing nearly 8,800 pregnancies in Catalonia, Spain, over more than a decade, the researchers concluded that the burden was not evenly shared. Women in more deprived neighborhoods, areas with higher unemployment, more temporary or manual work, and lower education levels, faced higher risks of developing GDM when exposed to high temperatures. The authors suggest this may be because women in these neighborhoods often face greater heat exposure and have fewer ways to protect themselves, whether due to poorer housing, less access to green or cool public spaces, or jobs that involve working in hot conditions.

Some studies suggest that the timing of heat exposure during pregnancy also matters. In eastern China, an analysis of over 3,000 pregnancies revealed that when temperatures climbed above 25°C, the risk of GDM increased most sharply between the 13^th and 18^th weeks of pregnancy, with a clear peak around week 16.⁶ Wider gaps between daytime and nighttime temperatures further raised the risk.

Researchers have observed similar patterns in larger populations. In southern California, they analyzed almost 396,000 health records over more than a decade.⁷ Extreme heat between the 11^th and 16^th weeks of pregnancy was associated with a higher GDM risk, while extreme low temperatures between the 20^th and 24^th weeks also increased the risk. The team found that these effects varied by location, with local factors such as greenness, tree cover, built surfaces, and land temperature either amplifying or reducing the risk.

How Heat Throws Blood Sugar Off Balance

As the evidence builds around the link between heat and GDM, scientists are also identifying the biological mechanisms behind this association. Teyton, who led the southern California study, said, “High temperatures may make the body less responsive to insulin, which can lead to insulin resistance.”⁸ This makes it difficult for the body to move sugar out of the bloodstream and into cells, causing blood sugar levels to rise. In hot conditions, the body also sends more blood toward the skin to release heat. This can affect how the body regulates glucose, though the exact mechanisms are still being studied.

Some effects may be indirect. During extreme heat, people tend to stay indoors, which can reduce their vitamin D levels. Vitamin D supports the body’s response to insulin, helping cells take up glucose from the bloodstream and regulate blood sugar levels.⁹ Heat exposure can also stress the body, triggering low-grade inflammation, which can interfere with how the body responds to insulin.

For Granés, the most likely explanation lies in how temperature affects the body’s ability to manage blood sugar. In controlled experiments, people exposed to cooler conditions for 10 days showed improved insulin sensitivity.¹⁰ Their bodies were better able to use insulin to regulate blood sugar. This has been linked to increased brown adipose tissue activity, a type of fat that burns energy to generate heat, helping the body use glucose more efficiently.

By contrast, higher temperatures may have the opposite effect. Epidemiological studies, she said, have also linked higher temperatures to increased rates of prediabetes, diabetes, and insulin resistance.¹¹

To examine these processes directly, many researchers have looked to animal studies. For example, researchers at Sokoine University of Agriculture exposed both pregnant and non-pregnant Wistar rats to sustained heat stress of 41–42°C for 21 days.¹² They found that pregnant rats had higher blood sugar, lower insulin levels, and signs of damage to pancreatic β-cells, which are the cells that produce insulin. This suggests that heat stress could promote the development of GDM.

The team also found that sustained heat can trigger a cascade of stress responses. “One of the central mechanisms is oxidative stress, where heat increases the production of reactive oxygen species, overwhelming the body’s antioxidant defenses,” study coauthor Robert Max, a biochemist at Sokoine University of Agriculture, explained.

Reactive oxygen species are unstable molecules that can harm cells. Antioxidants help keep them under control. This imbalance, he added, can damage pancreatic β-cells, reducing the body’s ability to control blood sugar. “Heat stress alone, even in the absence of other major metabolic challenges such as a high-fat diet, was sufficient to induce…disturbances associated with GDM,” he said.

Managing Risk in a Warming World

As Sanjana Tasgave* from western Maharashtra started facing unusually high temperatures in February this year, she began to feel unwell. At first, she dismissed it as a normal part of pregnancy. But within days, swelling in her legs and face became severe, and she began to feel breathless. Her family rushed her to the hospital.

In about eight to 25 percent of GDM cases, people can develop polyhydramnios, which is a dangerous rise in amniotic fluid, the liquid that surrounds and protects the baby in the womb.¹³ When blood sugar levels are high, extra sugar passes to the baby, triggering increased urine production. This raises fluid levels, and the uterus stretches beyond its normal capacity, causing breathlessness, discomfort, and restricted movement. It can lead to complications like early labor and excessive bleeding after birth.

Tests showed that her blood sugar levels were extremely high, with fasting values crossing 240mg/dL, far above the recommended level in pregnancy, which is typically kept below about 95mg/dL. Doctors began insulin treatment.

They advised Tasgave to rest, stay hydrated, and avoid the heat. What struck her family was how common such cases were. At the hospital, Tasgave’s mother met several other women facing similar complications. “I had never experienced such heat in my entire life. Every year, it feels like there’s a new record, which scares me about what the future holds,” Tasgave said.

Ashwini Talwar, a community healthcare worker from Maharashtra’s Arjunwad village, supported Tasgave during her pregnancy. “Over the years, as temperatures have increased, I’ve seen a clear rise in cases of GDM. Earlier, this wasn’t as common,” she said.

Scientists said these risks can be reduced to some extent. Teyton pointed to individual and public health measures like staying hydrated, avoiding direct sunlight, and limiting physically demanding activities. Cooling the body, whether through damp cloths, showers, or keeping indoor spaces shaded and well-ventilated, can also help.

Beyond this, she pointed to broader changes like opening cooling centers, issuing targeted heat alerts for pregnant people, and modifying neighborhoods. Increased greenness, she said, could reduce the impact of extreme heat on GDM risk.

Granés said heat risks need to be communicated carefully, without overwhelming pregnant people. She also stressed the importance of designing cooler living environments. Rather than relying solely on air conditioning, she urged, “building with principles of energy efficiency and passive cooling in mind.” At the same time, she acknowledged that many solutions reflect urban, high-income settings.

For women like Patil, these solutions are not always easy to put into practice. Even now, the difficult experience has stayed with her. “I never imagined the heat could affect my pregnancy like this,” she said. As temperatures continue to rise each year, she worries about what this could mean for other women like her.

She added that for many, these hardships go unnoticed. Their struggles are absorbed into what is often seen as a “normal” part of pregnancy, she added.

*Tasgave’s name has been changed to protect her identity as in her village conditions like GDM are often considered sensitive.