Study highlights increased risk, especially in Western states and during intense smoke events

Exposure to wildfire smoke during pregnancy was associated with a higher risk of preterm birth, especially in the Western United States and during periods of intense or prolonged smoke, according to a study funded by the National Institutes of Health’s Environmental influences on Child Health Outcomes (ECHO) Program.

Wildfires are becoming more frequent and severe, raising concerns about their impact on public health. While previous studies suggested a link between wildfire smoke and preterm birth, ECHO’s research built on this by studying a diverse population from prospective cohort sites nationwide. Researchers also looked closely at the role of exposure intensity, duration, and timing.

“Our study adds to growing evidence that wildfire smoke is a concern for maternal and child health. Incorporating wildfire smoke considerations into prenatal care and public health guidance could help expecting parents protect themselves during smoke events, especially in areas where wildfires are common,” said Allison Sherris, PhD, of the University of Washington.

Researchers analyzed data from over 20,000 births between 2006 and 2020 at 30 study sites across the U.S. They matched daily levels of air pollution from wildfire smoke (fine particulate matter, or PM_2.5) to each participant’s home address during pregnancy. They tracked both the average amount of smoke and the number of “smoke days,” as well as “smoke waves”—stretches of consecutive days with high smoke levels.

The team used statistical methods to compare pregnancies week by week and adjusted for many factors, including maternal age, race, ethnicity, neighborhood poverty, season of conception, birth year, and more.

Key Findings

• Widespread Exposure: Nearly all pregnant participants (99.2%) experienced at least one day of wildfire smoke, with an average of 22 smoke days during pregnancy.
• Preterm Birth Risk: The overall rate of preterm birth was 8.4%, with higher rates among Black (11.7%) and American Indian or Alaska Native (13.5%) participants compared to White (7.6%) and Asian, Native Hawaiian, or Pacific Islander (8.0%) participants.
• Link Between Smoke and Preterm Birth: For every 1 microgram per cubic meter (µg/m³) increase in average wildfire PM2.5 during pregnancy, the odds of preterm birth increased by about 7%. The association was even stronger in the Western U.S., where wildfire smoke is more intense and frequent.
• Intensity and Duration Matter: The risk of preterm birth was higher with more intense smoke and longer-lasting smoke events. The strongest associations were seen for mid-pregnancy exposure to lower levels of smoke and late-pregnancy exposure to high-intensity smoke days.
• Critical Windows of Vulnerability: Certain periods during pregnancy—especially mid-pregnancy for lower-intensity smoke and late pregnancy for high-intensity smoke—were linked to the greatest risk increases for preterm birth.

“Preterm birth has strong implications for future child health. Better understanding of how to prevent preterm birth will pay off as a healthier population into the future,” said Catherine Karr, MD, PhD, of the University of Washington.

The study points to the need for more research on why some regions or groups may be more affected, how wildfire smoke combines with other risks like extreme heat, and which prevention strategies work best. It also highlights the importance of public health agencies and healthcare providers offering ways to help keep pregnant people safe during wildfire events.

This collaborative research was published in The Lancet Planetary Health.

About ECHO
The ECHO Cohort Consortium is a research program supported by the National Institutes of Health (NIH) with the mission to enhance the health of children for generations to come. ECHO Cohort investigators study the effects of a broad range of early environmental influences on child health and development. For more information, visit echochildren.org.