Exposure to PM2.5 during pregnancy, particularly in the earliest weeks, may reduce infant birth weight, according to a new cohort study in the US.
The research, based on nearly 17,000 mother–newborn pairs, found that higher exposure was associated with lower birth weight relative to gestational age. The strongest effects were observed during early pregnancy, a period when key developmental processes are underway.
While previous studies have linked prenatal PM2.5 exposure to reduced birth weight, most averaged exposure over the entire pregnancy, potentially obscuring periods of particular vulnerability.
In this study, researchers used daily residential PM2.5 estimates across the United States and examined exposure week by week throughout pregnancy. They applied advanced statistical models to identify both cumulative effects and critical windows when exposure had the greatest impact.
Exposure to PM2.5 during pregnancy was linked to lower infant birth weights, according to the study. This was particularly true for a critical window in the first five weeks of pregnancy.
The findings also revealed a sex-specific difference: this early-pregnancy effect was observed only in male infants, suggesting that male foetuses may be more vulnerable to air pollution at this very early stage of development.
The study also identified regional differences. Negative associations between PM2.5 exposure and birth weight were observed in the Northeast, Midwest, and South, with critical windows ranging from early to mid-pregnancy.
No significant association was detected in the Western United States. Researchers suggest that these differences may reflect regional variation in particle composition, emission sources, or social and health-related factors.
By contrast, the analysis did not find differences in susceptibility by race or ethnicity. The authors caution that this does not rule out disparities, noting that structural factors and unequal pollution burdens remain well documented and warrant further study.
Although the observed reductions in birth weight were modest, the findings carry public health significance. Low birth weight – even among full-term infants – is associated with higher risks of neonatal complications and chronic health conditions later in life. Given that PM2.5 exposure is widespread, even small shifts in population-level birth weight distributions can have meaningful consequences.
The full research can be read here.
Photo Christian Bowen / Unsplash
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