Wildfire smoke linked to smaller babies in exposed areas

Exposure to wildfire smoke and extreme heat before or during pregnancy may increase the risk of babies being born smaller than expected, particularly in climate-vulnerable neighbourhoods, new research suggests.

The study found that women exposed to these conditions as early as the month before conception were more likely to give birth to small-for-gestational-age (SGA) babies – infants whose birth weight is below the 10th percentile for their stage of pregnancy.

Researchers examined 713 births from the MADRES cohort – an ongoing study of pregnant women in Los Angeles – between 2016 and 2020.

They tracked exposure to wildfire smoke and high temperatures across the preconception period and the first trimester.

The team from the University of Southern California found that greater exposure to wildfire smoke and heat during these stages was linked to higher risks of SGA births.

Roxana Khalili is a researcher in the department of population and public health sciences at the Keck School of Medicine of USC and lead author.

Khalili said: “We already know that poor air quality is associated with adverse health outcomes and that pregnant women and fetuses are especially vulnerable.

“Our knowledge about the specific effects of wildfire smoke during pregnancy has been limited. We know even less about the impact of these hazards right around or before conception.”

To estimate wildfire exposure, researchers used data from CalFIRE to identify fire events and calculated smoke density and ground-level particle pollution – including soot, black carbon and burned vegetation – using data from the NOAA hazard mapping system.

Exposure estimates were matched to each participant’s daily residential location.

Women exposed to moderate smoke-density days during their first trimester also had a higher likelihood of giving birth to low-birth-weight babies – defined as weighing less than five pounds, eight ounces.

Both SGA and low birth weight are associated with short-term complications and long-term cognitive, metabolic and developmental concerns.

Heat stress exposure was assessed using detailed meteorological data on temperature, humidity, and wind speed, along with additional modelling to account for sun angle and cloud cover – a method designed to reflect how hot days feel outdoors.

Living in climate-vulnerable neighbourhoods – areas experiencing high environmental stress and limited resources – nearly doubled the effect of preconception heat stress on the likelihood of SGA births.

Khalili said: “Where you live makes a difference in your health.

“So does the timing of your exposure during or immediately before pregnancy.

“We wanted to better understand how these risks might differ for women who lived in neighbourhoods that experience more climate-related stressors and have fewer resources to deal with them.”

The researchers also found that Fenton Z-scores – which measure how much an infant’s size deviates from the expected average – significantly decreased with more wildfire days during pregnancy.

Khalili said: “Overall, we found consistent associations between increased exposure to wildfire smoke and heat stress during preconception and the first trimester with the odds of having an SGA infant.”

The findings are especially relevant following prolonged wildfire events in Los Angeles in January 2025 and the increasing frequency of climate-related hazards across the US.