When climate-induced heat waves lead to outdoor safety advisories for children, older adults, and people who are sick—not to mention pets and livestock—researchers now say pregnant women should be included in the warnings.
Pregnant women “have traditionally fallen outside of our conception of who is vulnerable to heat,” said George Washington University sociologist Sabrina McCormick. “We need to really change that conception.”
That’s because a small group of researchers in the United States, Canada, and elsewhere “are methodically accumulating evidence suggesting that higher temperatures could be linked to a higher risk of premature births, stillbirths, or other negative pregnancy outcomes,” The Atlantic reports. While the preliminary findings raise new questions that can only be answered through further research, “enough evidence has already surfaced to warrant increased scrutiny—particularly as global warming is expected to drive average temperatures ever upward over coming decades.”
About a decade ago, Rupa Basu, chief of air and climate epidemiology at the California Environmental Protection Agency, noticed that research linking air pollution with negative pregnancy outcomes pointed toward a seasonal pattern, but had never been adjusted for temperature differences. She decided to look into it, matching more than 58,000 preterm births during the warm months of 1999 through 2006 with available temperature and air pollution data.
She discovered that a 10-degree difference in “apparent” temperatures—a measure like Canada’s humidex that combines heat and humidity—produced an 8.6% increase in premature births, independent of air pollution. When she studied 8,500 stillbirths over a decade of warm seasons, she found a 10.4% increase.
Researchers at Quebec’s Institut national de santé publique (public health institute) and the U.S. National Institute of Child Health and Human Development have come up with similar results. The latter study, covering 15 hospital referral regions from Los Angeles to Miami to Massachusetts, concluded that stillbirths were 3.7 times as likely when women experienced temperatures in the top 10% of the range for their location.
“It’s much higher than we would have thought,” epidemiologist Pauline Mendola told Atlantic correspondent Ellie Kincaid. “To see something with an odds ratio of three to four—that’s pretty striking.”
The article acknowledges limitations in the research to date that make it difficult to map the results back to individual women’s experience, and the biological mechanism leading to the vulnerability still isn’t clear.
But “I think it’s settled there’s an association,” said Rush University Medical Center obstetrician Gary Loy, who wasn’t involved with the study. Mendola adds that there are “lots of plausible ties” that could be tested in future research: pregnant women are less able to regulate their body temperatures, and stress from a rising temperature “could also trigger an inflammatory response that constricts a pregnant woman’s blood vessels,” The Atlantic notes, “making it harder for blood carrying oxygen and other essentials to get to the placenta and putting the baby at risk.” Maternal dehydration could reduce the amount of amniotic fluid in the womb, contributing to fetal death, and temperature-sensitive proteins in the blood could cause vessels to dilate, reducing blood pressure for the mother and blood supply to the fetus.
While those details will take time to tie down, McCormick said it’s legitimate to include pregnant women in heat warnings now. “I do think we have enough research at this point to be concerned about pregnant women as a vulnerable population,” she said, particularly because heat exposure—unlike some of the other factors that contribute to stillbirth—is something pregnant women can try to address by getting access to air conditioning and staying hydrated.