Extreme Heat Threatens the Health of Unborn Babies

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.

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