Pregnant people exposed to extreme heat are at higher risk of developing life-threatening complications during labor and delivery, according to new research published in the Journal of the American Medical Association on Thursday. The research adds to a growing body of evidence showing the impact extreme heat has on a pregnancy, while also making a distinction between long-term exposure and events like heat waves.

“I think this [distinction] is really important,” said Ashley Ward, director of the Heat Policy Innovation Hub at Duke University who was not involved in the study. “Most of the research around pregnant women has centered on acute events like a heat wave … but honestly, what we are all experiencing this summer is an excellent example of really what I would consider chronic heat exposure.”

The researchers used data provided by Kaiser Permanente Southern California, a health care network, to identify over 400,000 pregnancies in the region that took place between 2007 and 2018. They then looked at temperature data from that same time period and used the daily maximum temperature to figure out how many days of heat — characterized as moderate, high or extreme — pregnant people were exposed to throughout their pregnancy, breaking it down by trimester.

The study found significant associations between both short- and long-term exposure (usually defined as 30 days of more) of environmental heat during a pregnancy and severe maternal morbidity. Severe maternal morbidity is a term used by the Centers for Disease Control and Prevention to define 21 unexpected outcomes during labor or delivery that are considered near misses for maternal mortality. This could include cardiac arrest, eclampsia, heart failure, sepsis and ventilation. (The primary findings excluded whether a blood transfusion occurred during delivery, usually an indicator of severe maternal morbidity, because the data wasn’t detailed enough.)

Researchers found that a high exposure to extreme heat through the duration of the pregnancy or in the third trimester was associated with a 27 percent increase in these risks. Exposure to a heat wave during the last gestational week was also associated with an increased risk of life-threatening delivery complications.

Globally, this summer was the hottest on record. Phoenix broke a record when the temperature topped 110 degrees 31 days in a row. Dozens of other cities including in Texas, Louisiana and California have experienced their own record-breaking temperature streaks.

While the study did not find significant demographic differences when breaking down the data by race and ethnicity, it did find that people with lower education levels had higher heat-related risk of severe maternal morbidity, said Anqi Jiao, an author of the study. This finding points to possible mitigation strategies that doctors, nurses and others in public health could take.

Read the full article about extreme heat for pregnant women by Jessica Kutz at The19th.