Wildfire smoke — one of the fastest-growing sources of air pollution in the United States — significantly increases the risk of preterm birth, with exposure during the second trimester posing the greatest danger, according to a landmark University of Washington study that analyzed more than 20,000 births across the contiguous United States.
The findings are not theoretical. With wildfire season beginning across the Western United States and smoke regularly reaching cities from coast to coast, millions of pregnant women will face smoke exposure this summer. Researchers say the evidence is now strong enough to prompt tailored public health messaging and immediate protective action for pregnant people during smoke events.
What the UW Study Found
The study, led by the University of Washington's Department of Environmental and Occupational Health Sciences and published in The Lancet Planetary Health in November 2025, analyzed 20,034 births from 2006 to 2020 across the contiguous United States, drawn from the Environmental influences on Child Health Outcomes (ECHO) program — a federal research project focused on environmental factors in children's health.
The team, which included 42 authors nationwide, examined the intensity, frequency, and timing of maternal exposure to fine particulate matter (PM2.5) from wildfire smoke — and found consistent, significant associations with preterm birth:
- For every small increase (1 microgram per cubic meter) in average wildfire-related PM2.5 exposure during pregnancy, the risk of preterm birth rose by approximately 7%
- Exposure to high-intensity smoke days and longer consecutive periods of high-intensity smoke were tied to additional risk
- Second trimester exposure showed the strongest overall association with preterm birth risk
- Late pregnancy (third trimester) exposure to high-concentration smoke days was also significantly associated with preterm birth
- The risk was highest in the Western United States , where smoke concentrations and exposure days were greatest
"The second trimester is a period of pregnancy with the richest and most intense growth of the placenta, which itself is such an important part of fetal health, growth and development," said Dr. Catherine Karr, a UW professor of environmental and occupational health sciences and of pediatrics in the UW School of Medicine, and co-author of the study. "So it may be that the wildfire smoke particles are really interfering with placental health. Some of them are so tiny that after inhalation they can actually get into the bloodstream and get delivered directly into the placenta or fetus."
Lead author Allison Sherris, a UW postdoctoral researcher, noted: "Preventing preterm birth really pays off with lasting benefits for future health. We don't always understand why babies are born preterm, but we know that air pollution contributes to preterm births, and it makes sense that wildfire smoke would as well. This study underscores that wildfire smoke is inseparable from maternal and infant health."
| Study Finding | Data |
| Study size | 20,034 births (2006–2020) |
| Published in | The Lancet Planetary Health (November 2025) |
| Led by | University of Washington DEOHS |
| Risk increase per 1 mcg/m³ PM2.5 increase | ~7% higher preterm birth risk |
| Highest-risk exposure timing | Second trimester (placental development) |
| Additional high-risk window | Late pregnancy exposure to high-concentration smoke |
| Highest geographic risk | Western United States |
| Preterm birth rate in U.S. | ~10% of all pregnancies |
| What PM2.5 can do | Particles enter bloodstream; potentially reach placenta and fetus |
Why This Summer Matters for Pregnant Women Across the Country
Preterm birth — delivery before 37 completed weeks of pregnancy — affects approximately 10% of all U.S. pregnancies. It is the leading cause of infant illness and death in the United States, and it carries long-term health consequences for children, including increased risk of breathing problems, feeding difficulties, developmental delays, vision and hearing problems, and cerebral palsy.
Wildfire smoke is no longer a seasonal concern limited to Western states. In 2020, smoke from California fires reached the East Coast; in subsequent years, smoke from Canadian wildfires blanketed the Midwest and Northeast. The American Lung Association's State of the Air 2026 report confirms that wildfire smoke is worsening national air quality trends, exposing tens of millions of Americans — including pregnant women who may be unaware that the smoky summer haze outside their windows carries specific fetal risk.
The fine particles in wildfire smoke (PM2.5 — particles 2.5 microns or less in diameter) are uniquely dangerous because of their size. Unlike larger particulate matter that is filtered by the upper respiratory tract, PM2.5 penetrates deep into the lungs, enters the bloodstream, and, as the UW study's co-authors note, can be delivered directly into the placenta or fetus. The biological pathways remain under investigation, but the epidemiological signal — across 20,000 births and more than a decade of data — is clear.
What Pregnant Women Should Do Now
Dr. Karr advised pregnant people to monitor the air quality index (AQI) and take precautions whenever air quality is unhealthy for sensitive groups. The EPA's AirNow website provides real-time AQI data by ZIP code and sends air quality alerts. An AQI above 100 is considered unhealthy for sensitive groups — which the EPA identifies as including pregnant people — and an AQI above 150 is unhealthy for everyone.
Protective steps recommended by UW researchers and public health officials include:
- Stay indoors with windows closed when outdoor AQI from wildfire smoke is elevated
- Use air conditioning in recirculate mode rather than bringing in outside air
- Use a portable air purifier with a HEPA filter in the rooms where you spend the most time
- When going outdoors during smoke events, wear an N95 or KN95 respirator — standard surgical masks do not filter fine particles effectively
- Monitor your symptoms and contact your obstetrician or midwife if you experience increased shortness of breath, chest tightness, or any signs of early labor
Pregnant women in the second trimester should be especially vigilant, given the study's finding that placental development during this window is most sensitive to smoke-related disruption.
The Accountability Gap: No Federal Guidance Tailored to Pregnant People
Despite the growing body of evidence, there is no federally published, pregnancy-specific guidance for wildfire smoke exposure. The EPA, CDC, and FEMA all publish general wildfire smoke advice — but none specifically addresses what pregnant people should do differently, what trimester-specific thresholds to observe, or what emergency resources are available.
The UW researchers call explicitly for "public health guidance tailored to pregnant people during wildfire season, including access to clean indoor air and early warnings about smoke events." As wildfire season 2026 begins, that tailored guidance does not yet exist at a federal level — a gap that state and local health departments, as well as obstetric care providers, will need to fill with their own messaging.
Frequently Asked Questions
Does wildfire smoke really cause preterm birth?
A University of Washington study of more than 20,000 U.S. births found a significant association between wildfire smoke exposure during pregnancy and higher odds of preterm birth. For every 1 microgram per cubic meter increase in wildfire-related PM2.5 exposure, risk rose approximately 7%. The association was strongest during second-trimester exposure.
Which trimester is most dangerous for wildfire smoke exposure?
The second trimester showed the strongest association with preterm birth risk, likely because the placenta undergoes its most rapid development during this period. Late-pregnancy exposure to high-concentration smoke was also significantly associated with risk. First-trimester risk was less pronounced in this study.
What AQI level is unsafe for pregnant women?
The EPA considers AQI above 100 unhealthy for sensitive groups, which includes pregnant people. At AQI levels above 100, pregnant women are advised to limit outdoor exposure. At AQI above 150, everyone — including healthy adults — should reduce outdoor activity and minimize smoke exposure.
Do N95 masks protect pregnant women from wildfire smoke?
N95 and KN95 masks can filter fine particles and reduce inhalation exposure when outdoors. They do not provide the same protection as clean indoor air, but they significantly reduce exposure compared to no mask or surgical masks. They are not designed for extended wear and may increase breathing effort — contact your care provider about appropriate use.
Is wildfire smoke a risk in states outside the West?
Yes. Smoke from Western wildfires and Canadian wildfires regularly spreads to the Midwest, Southeast, and Northeast, degrading air quality far from the fire source. The ALA's State of the Air 2026 confirms wildfire smoke is now a national air quality problem, not a regional one.
