As concerns grow over the gradual warming of our planet due to climate change, one area of particular worry is the impact hotter temperatures will have on health. The risks for vulnerable people during heat waves has been widely researched. Public health experts have been warning for years that the elderly, infants, those living with illnesses, and even pets should stay out of the heat. 

But the risk for pregnant women and the fetuses they carry has largely been left out of the conversation—that is, until now, as emerging research shows just how dangerous extreme temperatures are for maternal and infant health. 

The hottest month on Earth, at least since temperature records began in 1880, was recorded this past July. While U.S. President Donald Trump might still be uncertain about the existence of global warming, the general population agrees that our earth is getting warmer. The number of days a year with a mean temperature above 90 degrees Fahrenheit in the U.S. is expected to increase from one to 43 by 2070. Exposure to this level of heat has already been proven to lead to adverse health outcomes, including hyperthermia, heat stroke, and death. 

In October 2019, Maya Rossin-Slater, assistant professor of health policy at Stanford University’s School of Medicine, along with colleagues Jiyoon Kim, assistant economics professor at Elon University, and Ajin Lee, assistant economics professor at Columbia University, released new research through the National Bureau of Economics that exposes the risks for maternal and infant health. They point to a number of dangerous—even fatal—outcomes for women and their babies linked to extreme heat exposure above 90 degrees Fahrenheit. 

“[I]t is time public health officials start including pregnant women as one of the populations most vulnerable to extreme heat and other climate related risks.”

Pregnant women are already known to have trouble adapting to changing temperatures. This new study adds to that understanding by finding that exposure to extreme heat increases the risk of maternal hospitalization during pregnancy. During delivery, mothers facing extreme heat exposure are more likely to experience hypertension and have longer hospital stays. Their children are in turn more likely to experience dehydration at birth, which is one of the leading causes of morbidity and mortality in infants. These infants are also more likely to be readmitted to hospital in their first year of life, usually because of prenatal jaundice, blood disorders, or respiratory diseases. 

Ultimately, heat waves experienced during pregnancy lead to worse maternal health and raise the probability of complications at childbirth. This new research adds to a small, but growing, body of research on heat exposure during pregnancy. Other studies have linked prenatal heat exposure to low birth weight (less than 5.5 pounds). The stress a mother faces battling hot temperatures can reduce blood flow to the uterus, thus depriving the fetus of oxygen.

What’s more, extreme heat experienced during the third trimester can lead to premature labor. On average, 25,000 preterm births occur each year due to heat exposure, totalling more than 150,000 gestational days of pregnancy lost annually. These numbers are expected to grow with rising temperatures.

Exposure to extreme temperatures in utero and during the first year after birth are associated with an increased risk of illiteracy, fewer years of education, lower standardized test scores, less adult annual earnings, and shorter stature later in life. 

All of these studies agree that extreme heat is very dangerous for pregnant women and their children. “One thing that really stands out to me is the consensus of the evidence. It’s quite strong and quite consistent,” says Sabrina McCormick in an interview with Lady Science. McCormick is an associate professor of Environmental and Occupational Health at the George Washington University, and has conducted a systematic review on the effect of heat on maternal and fetal health. “It’s very clear that pregnant women are at risk of a variety of problematic outcomes when exposed to heat, as are their fetuses.” 

The researchers in Rossin-Slater’s study used data from 2.7 million in-patient records in three states: Arizona, New York, and Washington. The team found that the risks related to prenatal heat exposure were significantly higher for black mothers, who were twice as likely to be hospitalized when exposed to extreme heat during their second trimester.

Rossin-Slater says this racial gap is influenced by where people live; black women, on average, live in areas with higher temperatures. It is also influenced by women’s access to mitigating resources, such as air conditioning, during heat waves. This racial gap in maternal health adds to an already shocking disparity in health outcomes between black and white women.

David Goodman, the maternal health team lead in the Division of Reproductive Health at the Centers for Disease Control and Prevention in Atlanta, says black women are already more than three times more likely than white women to die from pregnancy-related complications in the U.S. Not only are pregnant black women at a higher risk, but the causes and timing of their deaths are different than those of white women. For instance, black women are more likely to die from cardiomyopathy, which occurs within a year after giving birth.

As Goodman tells Lady Science, clinical factors explain only half the cases of pregnancy-related deaths. That’s why, he stressed, it’s important to consider non-clinical causes that have a disproportionate impact on black communities—racism, inequalities, unstable housing, access to insurance and healthcare providers. He says it’s necessary to look beyond clinical causes to ask whether “less access to care for some populations is due to past relations of disrespect or racism in the system.” 

Extreme heat is adding another health burden on black women who are already struggling to receive appropriate, equitable, and affordable reproductive healthcare in America. Beyond reversing global warming, the only adaptive measure researchers suggest is to get out of the heat and into an air-conditioned space. But this is harder for people living in poverty who cannot afford air-conditioning; black households in particular have less access to air conditioning. 

There is a chance that people can adapt to hotter temperatures. Heat shocks have tended to impact people who are not used to extreme temperatures. As Rossin-Slater notes in her study, “a 90-degree day in Arizona in August is in fact quite typical, but a 90-degree day in Washington state is much less typical.” Therefore, pregnant women’s health in Arizona and New York was less impacted by heat waves than that of women in Washington. 

“Extreme heat is adding another health burden on black women who are already struggling to receive appropriate, equitable, and affordable reproductive healthcare in America.”

Unfortunately, warmer temperatures aren’t the only health risk brought on by climate change. Rossin-Slater warns that a number of stressful events related to climate change can have a negative impact on maternal and infant health—including exposure during pregnancy to hurricanes or extreme storms, disruptions in clean water supply, increased environmental toxins, and air pollution.

We might have fewer pregnancies to worry about, however, because extreme temperatures also appear to be having an impact on fertility. One study found that extreme temperatures (above 80 degrees Fahrenheit) caused a large decline in birth rates eight to 10 months later. This decline could indicate that extreme heat is having an impact on fertility, and as a result, population reduction is yet another risk of climate change.

“I think it’s important for people to understand that climate change is directly making these exposures more dangerous,” says McCormick. She says it is time public health officials start including pregnant women as one of the populations most vulnerable to extreme heat and other climate related risks. 

Image credit: Pregnant woman surrounded by plants under the sun by Josh Bean via Unsplash