At the time of writing, millions of acres are burning across California, Oregon, and Washington state — fires that have been impacted by record-setting heat as a result of climate change. Rising temperatures and poor air quality have long been linked to adverse health outcomes including cardiorespiratory disease and cancer in nonpregnant adults. Newer data suggest that heat and air pollution may also be associated with adverse pregnancy outcomes according to a recently published systematic review of observational studies.

This systematic review evaluated 68 cohort, cross-sectional, time-series, and case control studies with over 32 million births in the U.S. between 2007 and 2019, and examined the effects of air pollution and heat on pregnancy outcomes, including preterm birth, low birth weight, and stillbirth. Exposure to air pollution was defined by the level of fine particulate matter (PM2.5) and/or the level of ozone and other pollutants. Heat was assessed using geographic temperature for the duration of pregnancy. Heterogeneity among studies precluded pooling of data and meta-analysis.

Exposure to PM2.5 was associated with increased risk of preterm birth (median 11.5 percent increase) in 19 of 24 studies, low birth weight (median 10.8 percent increase) in 25 of 29 studies, and stillbirth (median 14.5 percent increase) in four of five studies. Heat exposure was likewise associated with increased risk of preterm birth (median 15.5 percent increase) in four of five studies, low birth weight (median 31 percent increase) in three of three studies, and stillbirth in two of two studies. Preterm births increased with every 5.6 degrees Celsius rise in temperature in three studies in California. The associations between heat exposure and preterm birth and stillbirth were particularly high in Black and Asian women and in Black and Hispanic women, respectively.

Like the COVID-19 pandemic, the climate crisis will continue to exacerbate existing health inequities.

This study raises important concerns about the impact of the environment on pregnancy outcomes. Any conclusions are naturally limited by the observational nature of the study designs, which is an issue with any analysis where randomization is not possible. Nonetheless, this review underscores the relationship between heat and air pollution exposure and adverse pregnancy outcomes, and highlights the impact on women of color, especially Black women. Although the absolute rates of preterm birth, low birth weight, and stillbirth may seem low, these outcomes are additive, are not happening in isolation, and likely have implications that we may not see for many years. Like the COVID-19 pandemic, the climate crisis will continue to exacerbate existing health inequities.

Only recently has climate change been widely perceived as a crisis with downstream health consequences such as increased exposure to heat and pollution, altered vector-borne disease transmission, threatened water quality and food security, increased risk of mental illness, and mass migration of people. This review supports one hypothesis attributing the observed drop in the number of premature births around the world during COVID-19 lockdowns to a decline in air pollution and adds to a growing body of literature indicating that air quality and temperature can have adverse effects on obstetric outcomes. It also demonstrates how climate change can disproportionately affect people of color.

With more evidence that the consequences of climate change go beyond flooding and storm damage, perhaps climate change-associated health risks could be incorporated into public messaging.

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EBM Focus articles provide concise summaries of clinical trials most likely to inform clinical practice curated by the DynaMed editorial team.