More premature babies with heat waves

Heat waves slightly increase the rate of premature births, according to a new US study. This is the first time that this effect can be proven.

“We were surprised to see this effect on the risk of preterm birth,” explains the lead author of the study published in May in the journal JAMA Network OpenLindsey Darrow of the University of Nevada.

“We thought that the omnipresence of air conditioning would make the risk minuscule. The effect is small, 2%, but since all pregnant women are affected, it remains significant on a population level. And disadvantaged areas are twice as affected.”

The study analyzed 53 million births between 1993 and 2017 in 50 U.S. metropolitan areas, representing more than half of the nation’s births during that period. The risk of preterm birth was analyzed based on heat waves in the week before the birth. A heat wave was defined as four days above 97.5^e percentile of normal temperatures.

In this study, women from ethnic minorities as well as women who had not graduated from high school, two groups overrepresented in disadvantaged areas, were at greater risk of giving birth prematurely.

Premature births (before 37 weeks) are linked to an increase in complications. Parturients and babies are nine times more likely to be hospitalized in intensive care during premature births, according to a 2020 study from the National Institute of Public Health of Quebec (INSPQ).

The Reno epidemiologist wanted through this work to put an end to a debate that had been going on for more than a decade. “When we started talking more about heat waves being more frequent with climate change, we wondered what the impact of the phenomenon would be on pregnant women, who are more sensitive to stress factors,” says M.^me Darrow: But until now, only two large studies had attempted to answer the question, and they didn’t provide definitive answers. With more than 50 million births, we wanted to get to the bottom of it.”

Quebec study

A Quebec study published in 2014 in the journal Epidemiologyon 220,000 births in Montreal between 1981 and 2010, had only found an effect on preterm births (at 37 and 38 weeks of pregnancy), but not for premature births.

And an American study published in 2017 in the journal Environmental Health Perspectives on 200,000 births had identified an increased risk (11%) of preterm birth by taking into account only heat waves during the first trimester of pregnancy. However, this restriction risked distorting the analysis, since women did not necessarily live in the city where they gave birth at the beginning of their pregnancy. This 2017 study also described an even greater effect of cold waves during the first trimester.

Audrey Smargiassi, co-author of the 2014 Quebec study, believes that the American study confirms that the risk posed by heat for pregnant women is low but very real. “It may be worth targeting vulnerable populations, particularly people who do not have air conditioning,” says the epidemiologist from the University of Montreal.

M^me Darrow wants to continue his work with more detailed data from certain states. “We see that there is an effect, and that it is more significant for potentially disadvantaged environments. We now need to see the impact for women who already have a higher risk of prematurity in their pregnancy, because of life stress in general, poverty, difficult work, or genetic risks. There is not enough precision in the federal data, particularly regarding the presence of air conditioning in homes. But by working with data from certain states, such as California, we may be able to detect this effect. If the increase in risk is greater, we will not need millions of births to observe it.”

Metabolism

The other avenue of research is to better understand what biological factors are involved in the increase in prematurity with heat waves, says M.^me Darrow.

“There are several possible factors, but they need to be better understood,” she says. “Dehydration can reduce blood flow to the placenta, which could create contractions and influence the production of hormones that promote labor leading to delivery. Metabolic mechanisms that protect cells from excessive heat increase inflammation, also linked to the onset of labor. Membranes can be weakened and rupture under the effect of heat. Finally, heat stroke can exacerbate pregnancy-related illnesses, such as hypertension, and directly affect the health of the fetus.”

“There is definitely an element of stress,” says Nathalie Auger, also an epidemiologist at the University of Montreal and co-author of the 2014 study. “It’s quite physically difficult to be pregnant at 35 weeks, even 32. Any additional stress is extra.”

M^me Auger cites a type of protein produced when the human body experiences heat stroke (heat shock proteins). The production of these proteins, discovered in the laboratory in the 1960s in the Drosophila fly, is associated with a reduction in the energy available for other activities of the human body, according to a study by biologists at the University of California, San Francisco, published in 2019 in the journal Physiology.