Global warming and more days of extreme heat are exacerbating the health risks of pregnancy, particularly among African-American women, according to new Stanford-led research.

The maternal mortality rate among all women in the United States is already the worst of any industrialized nation. And black women are three to four times more likely to die from pregnancy-related problems than white women.

“It is truly a crisis that in America, one of the wealthiest countries in the world, more women are dying from pregnancy or childbirth complications than in any other developed country,” said Maya Rossin-Slater, a core faculty member at Stanford Health Policy and a faculty fellow at the Stanford Institute for Economic Policy Research.

In a new working paper published by the National Bureau of Economic Research, Rossin-Slater and two other health economists underscore how little research is out there about the impact of rising temperatures on the health of mothers and their newborns.

Pregnant women, for example, are not able to regulate body temperature as efficiently as non-pregnant individuals due to the physiological changes they undergo during gestation. Heat exposure can alter blood flow in the placenta, which can weaken the placenta and lead to complications. And high heat can lead to other pregnancy complications, such as hypertension, preeclampsia and prolonged premature rupture of membranes.

“All of these issues can translate into women needing to be hospitalized during pregnancy and experiencing complications during childbirth,” wrote Rossin-Slater, an assistant professor of health research and policy at Stanford Medicine. Her co-authors are Jiyoon Kim, assistant professor of economics at Elon University, and Ajin Lee, an assistant professor of economics at Michigan State University.

The researchers said most of the discussion about maternal health focuses on the health-care system, but that other determinants of poor maternal health and racial disparities are much less understood, particularly when it comes to how the environment is impacting pregnancy.

So they launched what they believe is the first study to identify the causal effects of prenatal exposures to extreme temperatures on the health of the mothers themselves.

As the Earth Warms, So Does Exposure to Extreme Heat

Their paper focuses on an environmental factor that is becoming increasingly relevant due to the growing consensus that climate change is contributing to a gradual warming of the earth: exposure to extreme heat.

The researchers studied the effects of exposure to extreme temperatures during pregnancy on maternal and child hospitalizations, using inpatient discharge records from three U.S. states with different climates: Arizona, New York and Washington. Their data comes from the State Inpatient Databases from the Healthcare Cost and Utilization Project, including 2.7 million inpatient records of 2.7 million infants and 2.2 million mothers in those three states.

And to measure temperature exposure, the researchers obtained data from the National Oceanic and Atmospheric Administration (NOAA).

For every county in their data, the researchers calculated the average temperature for every month. Then for every given day in a specific month in that county, they looked at the historic average for how high or low that day’s temperature was relative to the overall temperature in that month in that county.

For example, a 90-degree day in Arizona in September would not be classified as extreme heat since it’s relatively common. But a 90-degree day in New York would be, since temperatures that high are much less common. They classified “extreme heat” as a given day when the temperature is more than three standard deviations (3SD) above that historic county mean.

Then, they compared the outcomes of women who are of the same race giving birth in the same county and calendar month, but in different years. These women are likely similar in terms of their demographics and socioeconomic status, but may be exposed to different temperatures during pregnancy. For example, consider a black woman giving birth in November 2011 in Queens County, New York, and a black woman giving birth in November 2012 in the same county. If there were a heat wave in Queens in the August 2012, then the latter woman is exposed to more extreme heat during pregnancy than the former. 

The economists found that each additional day with heat that is at least 3SDs — or substantially higher than the historic county-month average — during the second trimester of pregnancy increases the likelihood that a newborn is diagnosed with dehydration by .008 percentage points.

“Our results provide new estimates of the health costs of climate change and identify environmental drivers of the black-white maternal health gap,” they wrote. “Understanding the health consequences of this increase in extreme heat is critical information for discussions about the costs of climate change and the possible benefits of mitigating policies.”

The researchers found that each additional day of extreme heat exposure during pregnancy increases black women’s likelihood of hospitalization during pregnancy. Since black women on average are exposed to more extreme heat than white women — due to different residence patterns and access to mitigating technologies like air conditioning — extreme heat may contribute to exacerbating the already large gap in maternal health between black and white women.

Detrimental Consequences of Rising Temperatures

Scientists predict global average temperatures will continue to rise over the next 50 to 100 years as greenhouse gases continue to trap more heat in the Earth’s atmosphere. The U.N. Intergovernmental Panel on Climate Change last year warned that nations worldwide must quickly reduce fossil fuel use to keep the rise in global temperatures below 1.5°C by 2050. 

The panel also said the number of days with mean temperatures above 32°C in the average American county is forecasted to increase from about 1 to 43 days per year by 2070-2099.

That could have detrimental consequences for babies and mothers alike.

“Overall, our findings on infant health suggest that exposure to extreme heat during the second trimester increases the likelihood of the baby being dehydrated at the time of birth,” the researchers wrote. “This, in turn, appears to increase the likelihood of subsequent readmission to the hospital many months later for causes linked to dehydration.”

And these impacts are typically missed when researchers only measure infant health using more standard variables, such as birth weight.

The authors note dehydration is one of the leading causes of morbidity and mortality in children. Studies show that children under 5 years old who have an average of two episodes of gastroenteritis associated with dehydration per year leads to 2 to 3 million pediatric office visits and accounts for 10% of all pediatric hospital admissions in the United States. 

Experts believe black women are three- to four-times more likely to die from pregnancy-related causes due to lack of access to and the poor quality of health care, as well as clinicians not monitoring black women as closely — or actually dismissing their symptoms altogether.

“The fact that the adverse impacts on health during pregnancy are larger for black than for white mothers suggests that climate change may exacerbate the already large racial gap in maternal health,” the researchers said.