Here’s how extreme heat is hurting pregnant women

As the world gets hotter, scientists are documenting a spike in preterm and still births, along with underweight babies.

On hot summer days in Kilifi, a largely rural coastal county in Kenya, air temperatures soar to 100 degrees Fahrenheit and rarely drop below 70. “This place has always been hot,” says medical anthropologist Adelaide Lusambili at the Aga Khan University in Nairobi, “but now it’s very hot.”

Like many other parts of the world, Kenya is witnessing a rise in annual temperatures, accompanied by more frequent, intense, and longer heatwaves and reduced periods of cooling. Such acute and sustained heat poses a particular problem for pregnant women, who are more vulnerable to it. As the world gets hotter, scientists are seeing a spike in stillbirths, along with early or underweight births.

Last summer, Lusambili and her colleagues interviewed pregnant women and new mothers, their family members, healthcare workers, and community leaders in Kilifi to understand how extreme heat was affecting maternal and newborn health. One healthcare worker told them that they were seeing more birth complications and more early births.

A growing body of scientific research supports this observation by suggesting hotter-than-usual daytime temperatures and warmer nights may be leading to an increase in adverse outcomes for pregnant women. These range from greater risks of stillbirth—when the baby is born dead after at least 20 weeks gestation—to higher chances of a preterm birth, in which the baby is born before 37 weeks rather than the full-term of 40-weeks.

Some studies suggest that higher temperatures led to more lower-weight newborns, which can result in health complications for the infant. A recent analysis of 70 studies set in 27 countries, including the United States, China, a few European, and Sub-Saharan African nations, showed that for every 1.8 degree Fahrenheit rise in temperature, the risks of preterm and stillbirths jumps by 5 percent.

“This might seem small,” says lead author Matthew Chersich, an epidemiologist at the University of Witwatersrand in South Africa, but the research points to a growing threat. With climate change fueling more frequent and intense extreme heat events around the globe, it is becoming clear that rising temperatures could increasingly endanger pregnant women, new mothers, and newborns.

How extreme heat impacts pregnancy

Although scientists haven’t been able to identify the periods during pregnancy when extreme heat poses the greatest risk, exposure to acute heat seems to cause problems both in the early and late stages.

What complicates all of these studies is that there aren’t unique signatures (such as a specific mutation) that can connect an individual stillbirth or preterm birth to an extreme heat event, says Lyndsey Darrow, an epidemiologist at the University of Nevada in Reno.

Researchers have used long-term datasets to match pregnancy durations and dates with temperature data to understand how much and which aspects of heat—its intensity and or duration—are harming this vulnerable segment of society.

Several studies suggest pregnant women may be at high risk of heat stress closer to their due date. Exposure to extreme temperatures in the final days or weeks of pregnancy can jack-up the odds of a stillbirth or trigger preterm delivery—which can raise the chance of respiratory illnesses, neurodevelopmental impairment, and death in early childhood. Other research indicates that when a mother endures high temperatures early in a pregnancy, the developing baby is at risk of heart, spinal cord, or brain defects, which may explain preterm or stillbirths. Some studies note that women may be vulnerable to acute heat during their entire pregnancy, not just the final stretch. 

So far, scientists haven’t determined the precise physiological pathways through which heat affects pregnancy, but they have a few hypotheses.

A pregnant woman’s body temperature tends to be slightly higher than average, and it can rise when surrounding temperatures spike. Because these women are more likely to be dehydrated and thus sweat less, they’re less able to cool themselves quickly, which is one reason why higher temperatures are so dangerous for this population and why they could induce fetal defects.

Dehydration can also thicken the blood, increasing the mother’s blood pressure and reducing blood flow, and thus delivery of oxygen and nutrients, to the baby. This can lead to underweight babies or premature birth.

Heat stress could also induce inflammation at the decidua—the maternal portion of the placenta— triggering preterm labor. Based on animal studies, elevated temperatures can stimulate higher levels of pregnancy hormones like oxytocin, which could initiate early labor.

While these hypotheses seem plausible, there’s still a lot we don’t know about whether and to what extent these mechanisms directly or indirectly impact a pregnant woman’s body when enduring extreme heat, says Britt Nakstad, a neonatologist at the University of Botswana.

Building data points and theories

In a June 2022 study, epidemiologist Darrow and her colleagues used fetal death records spanning 1991 to 2017 from six U.S. states—California, Florida, Georgia, Kansas, New Jersey, and Oregon—and found a 3 percent increase in stillbirth risks when pregnant women experienced four consecutive hot days the prior week. When temperatures exceeded 95 degrees, these risks increased slightly.

In North Carolina, researchers found nighttime temperatures closely associated with preterm births. Between 2011 and 2015, they found that the risk of preterm birth increased by up to 6 percent with every two-degree bump in temperatures exceeding 75 degrees between May and September.

“What we might be seeing is an inability to recover from exposure during the day,” says Ashley Ward, a climate health scientist at Duke University and lead author of the North Carolina study. “We’re having increasingly warmer nights, and that’s where we should be concerned.”

In another study that investigated the effects of heat on women in 14 low- and middle-income countries including Ethiopia, Nigeria, Nepal, and South Africa, researchers noted greater odds of preterm and stillbirths when pregnant women experienced extreme temperatures and warmer, humid nights during the seven days prior to birth.

While such heat waves and acute temperatures are dangerous, an overlooked long-term factor includes a gradual increase in average seasonal temperatures, which can also threaten pregnant women and their babies. As with many other health and economic inequities—including access to shade, fans, and air-conditioning—some women are a lot more vulnerable to this heat stress than others.

Not everyone is impacted equally

In the U.S., heat-related adverse pregnancy outcomes are nearly double for Black and Hispanic women compared with white women. This isn’t surprising given that women of color tend to live in densely populated, built-up neighborhoods that heat up quickly and take longer to cool because of the lack of green space. It’s also likely that many of them can’t afford or lack access to air conditioning on extremely hot days.

Similarly, researchers studying maternal health in lower- and middle-income countries suspect that the impacts of climate change-linked heatwaves and rising temperatures on pregnant women residing in these regions will be far greater. In these parts of the world, women with limited nutritious food continue to engage in exhausting household chores—from walking long distances to fetch water to farming and gathering firewood—late into their pregnancy during acute heat.

While a majority of the research in the field is skewed to heat impacts on pregnant women residing in high-income countries, scientists like Lusambili and Chersich are trying to change that. Lusambili’s team is engaging with groups within the Kilifi community to boost awareness about the risks of extreme heat and the importance of reducing workload on pregnant women. Chersich hopes to develop an alert system that can warn pregnant women to take precautions on hot days.

In the U.S., such conversations about heat exposure and its impacts on pregnancies remain limited. “We haven’t done a great job of training [healthcare] professionals to talk to their patients about heat risks,” Ward says. Efforts are lagging in communicating ways to minimize heat exposure like staying indoors, using air conditioning, when people have the means, or accessing cooling centers.

“We are now essentially having climate change that is synonymous with being a hot tub,” says Veronica Gillispie-Bell, an obstetrician gynecologist at Ochsner Health in New Orleans. As healthcare providers taking care of pregnant individuals, she says, we need to do more to understand the risks and work harder to provide solutions.

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