Expert Commentary

Extremely hot weather during pregnancy poses health risks to mothers and infants

Exposure to extremely hot weather relative to the norm during pregnancy puts women and infants at risk for health problems.


Exposure to extremely hot weather relative to the norm during pregnancy puts women and infants at risk for health problems, finds a new study of maternal health in three states with varying climates.

For mothers, extreme heat increases the likelihood of hospitalization during pregnancy and hypertension during childbirth. For infants, the risks include dehydration at birth and being hospitalized in the first year of life.

Black mothers are especially vulnerable, according to a new working paper from the National Bureau of Economic Research, “What to Expect When It Gets Hotter: The Impacts of Prenatal Exposure to Extreme Heat on Maternal and Infant Health.”

The findings have sobering implications for maternal health trends, particularly as global temperatures rise.

“Given that extreme heat days are rising steeply in frequency and also in severity in just the next few decades, I think this sheds light on what’s going to happen to mothers’ health, and also the health disparities between black women versus white women,” says Jiyoon Kim, an assistant professor of economics at Elon University. She co-authored the paper with Ajin Lee, assistant professor of economics at Michigan State University, and Maya Rossin-Slater, assistant professor of health research and policy at Stanford University School of Medicine.

The paper looks at data collected on all maternal and child inpatient hospitalizations in three states with dissimilar climates — Arizona, New York and Washington. The data include records corresponding to a total of 2.68 million infants and 2.24 million mothers spanning from 2003 through 2013.

Kim explains that the project was motivated by her own experience as a mother.

“I had to move from Indiana to Florida in the middle of my pregnancy,” she says. “And then that led to an extreme weather change during my pregnancy period. That actually motivated this research question, personally, to me, to see whether there is any effect of extreme weather shocks during pregnancy — on my health, as well as on the infant’s health.”

The researchers matched corresponding county-level weather data — including average, maximum and minimum ground temperature and precipitation levels — from the National Oceanic and Atmospheric Administration (NOAA) to the hospitalization data.

They focused on relative temperature shocks — that is, unexpected variation in weather given the area’s average climate. They considered days in which a county’s average temperature was at least three standard deviations above the monthly average to be an extremely hot day. Standard deviation indicates the variation of a given value from the average. The more standard deviations away from the average a value is, the farther it is from that value. For that reason, standard deviation is a helpful tool to indicate relative extremes in temperature.

Kim adds that another personal experience influenced the study design. While her first pregnancy did not overlap with the summer months, her second did. Kim wondered whether this kind of prolonged, seasonal weather exposure might also have an effect on maternal and infant health.

She explains that it’s difficult to study these questions through a randomized experiment. Randomized, controlled trials are generally considered the gold standard of medical studies because their design — in which an intervention is randomly assigned to some participants in a trial and tested against a control group, which receives a standard treatment or a placebo — can provide evidence of causation. Where people live isn’t randomly assigned, and people can control when and whether they conceive.

With these considerations in mind, the researchers designed their study such that the findings point at a causal association between weather variation and health outcomes. They compared health outcomes among women of the same race and ethnicity who lived in the same county and gave birth in the same month. The key difference: the year they gave birth.

“The only difference between these two women is the year of birth,” Kim explains. “That could basically allow us to see the difference in their birth outcome by looking at the weather changes between these two years.”

The researchers find that for each additional day a pregnant woman spent in extreme heat during her second trimester, the odds  of her being hospitalized increased by 4.8%. For each additional day of extreme heat she endured during the third trimester, the likelihood that she was hospitalized increased by 3%. The increase in hospitalizations was mainly driven by pregnancy complications, including hemorrhage, high blood pressure and early labor.

Effects were larger for black women than white women. The likelihood of hospitalization for black women experiencing an additional day of extreme heat during any trimester increased by 5%, whereas for white women, the likelihood of hospitalization increased by 2.6%.

Extreme heat during pregnancy had effects on maternal health at childbirth, too. The researchers find each additional extreme heat day in the first trimester increased the probability of a complication at childbirth by 1%. An additional day of extreme heat during the third trimester increased the likelihood a mother would be diagnosed with high blood pressure at childbirth by 2.9% and increased the mother’s length of stay at the hospital by 0.3%.

Infants’ health was also affected by extreme heat. For each additional extreme heat day in the second trimester of pregnancy, the likelihood that a newborn was diagnosed with dehydration increased by 31%. The probability the infant was readmitted to the hospital within his or her first year of life increased by 3.4%. These re-hospitalizations were commonly caused by jaundice, blood disorders and respiratory diseases, which research links to dehydration.

There were not disparities in infant health by race or ethnicity, “which was pretty surprising because we were expecting to see a similar disparity in infants’ health,” Kim says. “That is the area where we’d like to investigate further in the future; why we do not see the disparity in infants but only for the mothers?”

The authors conclude: “Our findings suggest that, in the absence of mitigating interventions, the projected increase in exposure to extreme heat over the next century may contribute to further worsening of maternal health. Moreover, since black women are both more likely to be exposed to extreme heat … and experience larger adverse impacts of heat exposure on pregnancy-related health, our estimates imply that climate change could further exacerbate racial disparities in maternal health.”

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