In the midst of an unseasonal California heat wave last late spring, Nathaniel DeNicola, an obstetrician-gynecologist at Hoag Hospital in Newport Beach, had an unusual case on his hands: A patient who had been carrying a perfectly healthy pregnancy for 32 weeks was going into early labor. It didn’t make sense; nevertheless, the baby was coming. The patient’s waters had broken, the baby’s heartbeat was dropping fast, and the child was in the breech position. The mother had an emergency C-section. After spending a couple of weeks in the neonatal intensive care unit, the baby was allowed to go home.
After the scramble to deliver the baby, DeNicola searched for reasons that might explain the premature arrival. Sometimes there are obvious causes for the early rupturing of membranes, like a chlamydia infection or a condition called cervical insufficiency, in which the cervix starts to dilate on its own. But those explanations didn’t fit DeNicola’s patient. Struggling, he settled on a different explanation: the searing heat. “I can’t prove that that was because of extreme heat; it’s very tough to assign that,” he says. But from his research, he knew that heat can trigger preterm births. And in his 12 years as a clinician, he has often seen more obstetric emergency visits during heatwaves.
Doctors have known for some time that certain groups of people, like the elderly and children, are particularly vulnerable to heatwaves. But in recent years, a new population has come into focus: pregnant people and their unborn babies. As the world warms up, there is a growing corpus of evidence that the heat is interfering with pregnancy, perturbing the delicate fetus in the womb, with the potential for serious complications.
And it’s plenty hot now. July 3 was the hottest day ever recorded globally. July was declared the hottest month on record. California’s Death Valley recently reached 128 degrees Fahrenheit—just two degrees short of a record for the entire planet. In Phoenix, Arizona, the daytime temperature hasn’t dipped below 110 degrees Fahrenheit in almost a month. In parts of the world, such as Iran, the heat index is teetering toward the threshold of what the human body can tolerate. Swathes of Europe are on fire.
Understanding the effect of extreme heat on pregnancies will require a major shift. Due to ethical concerns, pregnant people have typically been excluded from studies of the effects of heat on physiology. (A recent paper drily pointed out that far more research has been done on the effects of heat stress on livestock “due to its economic importance.”) It means that much of what we know comes from animal studies.
So far, there are many theories, but not many firm conclusions. Animal studies have shown that heat can provoke an increase in the secretion of oxytocin, a key hormone involved in labor, which may also explain the phenomenon in humans. It could be that extreme heat triggers the premature rupture of membranes, leading to a too-early birth. Or it could be that heat strain causes the release of inflammatory proteins, prompting preterm labor. Maybe it’s dehydration caused by the heat, causing the release of prostaglandins, lipids that will trigger contractions, and these contractions can be so intense that the body goes into early labor.
In pregnant people, we know that the body alters the way it regulates its temperature to accommodate for increased body size and the metabolic toil required to grow a baby. This, in turn, limits the body’s capacity to dissipate heat. So when it’s super hot, a pregnant person is perhaps less equipped to deal with the heat. Another theory is that when a pregnant person is experiencing heat stress, the body releases heat-shock proteins, which could trigger physiological reactions that are harmful to the baby and its bearer.
A few papers have tried to pinpoint how heat affects the development of human babies, notably a 2022 study that followed 92 pregnant farmers working in The Gambia in West Africa. Led by Ana Bonell, a research fellow at the London School of Hygiene & Tropical Medicine, the team wanted to figure out how doing agricultural labor out in the heat was affecting their pregnancies. Bonell knew that heat stress can trigger the release of cortisol, which could hinder blood flow to the placenta. So her team decided to measure stress, both on the mother and the fetus. Alarmingly, they saw that for every 1 degree Celsius increase in heat stress (as measured using the Universal Thermal Climate Index), there was a 17 percent increase in fetal stress—defined as abnormally high heart rate or slower blood flow through the umbilical cord. Overall, they concluded that heat strain on the mother translated into strain on the fetus.
Bonell felt it was important to be doing the research in a region likely to face some of the worst tolls of a heating world, where the typical escapes—say, retreating to an air-conditioned building—aren’t available to all. “There’s a massive inequality and climate justice agenda that goes with any research around climate change,” she says. “West Africa is one of the most vulnerable to the impacts of climate change. It just felt right, really.”
Strain on the fetus can have serious consequences. Multiple studies have found that even small increases in ambient temperature can increase the risk of preterm birth: A study from California found that for every 5.6 degrees Celsius increase in ambient temperature exposure, the risk of a preterm birth increased by 8.6 percent. Another analysis also found that the risk of preterm birth increased as temperatures went up.
Several studies have also found significant links between heat exposure and low birth weight. A 2022 study conducted in Massachusetts found that higher ambient temperature resulted in smaller babies. One reason why, the paper proposes, is that perhaps the induced heat-shock proteins mess up normal protein synthesis, which may wreak havoc with the development of the fetus’s organs.
For some babies, the heat can prove fatal. Another recent analysis led by Bonell reviewed the link between heat exposure and stillbirths: One study from Western Australia found that the risk of stillbirth increased by 41 percent if the mother was exposed to moderate heat stress—around 32 degrees Celsius—in the last two weeks of pregnancy.
And hot weather may affect a baby’s development in other ways: A 2021 analysis found a higher incidence of anomalies, such as cardiac defects, spina bifida, or cleft lip, at higher temperatures. A 2019 study took existing research that linked heat exposure to congenital heart defects and extrapolated how many such cases we can expect in the coming years: The authors estimated that over an 11-year period, an additional 7,000 babies will be born with congenital heart defects in the eight US states they studied. According to Bonell, there’s also early evidence from animals that heat stress may be triggering epigenetic changes linked to long-term adult chronic diseases such as heart disease and diabetes.
And in the United States, any potential harm to a fetus brings with it other concerns. Pregnant people, more than ever in a post-Roe world, are at risk of criminalization for behavior that may harm the fetus, particularly in states that recognize fetal personhood, which grants legal rights to a fetus from conception. “You might live in a state that criminalizes your adverse birth outcome because you took a walk when it was hot, or you were working outdoors when the temperatures were too high,” says Adelle Dora Monteblanco, an assistant professor of public health at Pacific University in Oregon.
As the research linking extreme heat to pregnancy complications piles up, public health bodies, including the World Health Organization and UNICEF, have started to take notice and include pregnant people and their unborn babies in warnings during extreme heat—although some, like the United Kingdom, are still leaving pregnant people out.
While including pregnant people in public health messaging is important, doctors also need to do more to keep patients fully informed on how to stay safe. “We know enough to act,” says DeNicola. “While we don’t have perfect solutions, we do have counseling we can give.” He knows to tell his patients to drink more water, and try to access any kind of cooling, if they have the means. If air-conditioning isn’t an option at home, people should look for cooling centers, or shopping malls, movie theaters, or libraries.
Research has shown that more targeted advice is still a necessity. A 2022 paper concluded that current guidance for pregnant people with regard to heat exposure is “sparse, inconsistent, and not evidence based.” Big questions remain, like at what stage in the pregnancy is the mother and fetus most at risk, or at what temperature conditions shift from risky to dangerous.
Yet there are limits to simply giving out advice. As the world heats up, pregnant people and their babies will continue to be vulnerable to these risks—particularly people from low-income households and people of color. Bonell points out that what people really need is practical help or the tools to help themselves. “You need some other solutions that aren’t just about education,” she says.
Updated 8-15-2023 5:00 pm BST: The story was corrected to state that increase in fetal stress outlined in the 2022 Lancet study was measured against the Universal Thermal Climate Index, not temperature.
