Exposure to heat in early pregnancy linked to increased health risks, study finds

Like many big cities, this summer in Toronto was one of the hottest on record, with six heat waves and 14 days above 90 degrees Fahrenheit (30 degrees Celsius). “I basically stayed indoors as much as I could,” says Sarah Peterson, a Toronto resident who spent this summer pregnant with her first child. “Even just walking outside for five to ten minutes was hard.”

But being pregnant during the summer isn’t just uncomfortable, it can also increase your odds of maternal health risks. That’s according to a study published on Aug. 19  in The Journal of Climate Change and Health.

Researchers at the Icahn School of Medicine at Mt. Sinai found that people who were in their first trimester of pregnancy during the summer months were more likely to develop high blood pressure, urinary infections and preeclampsia during pregnancy. Preeclampsia — persistent high blood pressure which normally develops after 20 weeks of pregnancy — is one of the leading causes of maternal mortality.

The study analyzed records of 819 pregnant women who received prenatal care in New York City from 2009 to 2014. Researchers partly based their findings on data from the New York City Heat Vulnerability Index, an interactive map that determines which neighborhoods are at greatest risk of adverse outcomes from extreme heat. 

Among members of the cohort who were in their first trimester during the summer months, those living in heat vulnerable neighborhoods had higher rates of preeclampsia than those who lived in less heat vulnerable areas; 19.2% compared to 11%.

“We need to be doing more to support people, especially those who are living in these high vulnerability neighborhoods,” says lead author Melissa Blum, a Mount Sinai medical student and Queens College researcher focusing on environmental exposures and pregnancy outcomes. 

Blum’s study found that neighborhoods with higher vulnerability to heat also had higher poverty rates and a higher proportion of Black residents. “We see that there’s racial disparities  in maternal outcomes,” says Blum, “so that’s really a problem that we need to be addressing.”

The study points out that people who live in cities, no matter the neighborhood, face added risks from the urban heat island effect, where buildings and roads trap heat and reduce nighttime cooling. It’s a phenomenon that’s bound to intensify as average temperatures increase, according to the Intergovernmental Panel on Climate Change. 

For Toronto resident Sarah Peterson, public transit is where she would like to see improvements. After she noticed some trains didn’t have functioning air conditioning, she started driving to work. “It didn’t feel safe to be packed in a rush hour train in 100 degrees Fahrenheit,” she says. “I wish the city treated air conditioning on public transit as a health issue.” 

Blum also wants to see pregnant people factored into heat mitigation plans, such as providing “more trees, more parks, cooling centers and better access to air conditioning.” 

For example, New York City’s Cooling Assistance Benefit helps low income households buy and install air conditioners. Blum notes, however, that funding is limited and tends to run out at the beginning of every summer. 

Caroline Li-Maloney, a PhD candidate studying women and heat stress  at the University of Ottawa, says one of the limitations of study is that extreme heat events weren’t assessed individually. “If you’re just looking at a set exposure from June to August, you’re not actually tracking when that person is exposed to extreme heat, you’re just assuming that during that point they’re going to be exposed to extreme heat.” 

Blum says her lab has plans to look at meteorological data to get a more granular picture in the future. As noted in the paper, “the continuing threat of climate change makes this area of research an urgent public health priority.”