Air pollution continues to be one of the world's most serious environmental health issues affecting almost all people on the planet. Exposure to dirty air during pregnancy has been associated with birth defects, including low birth weight and premature delivery. Now, a population-based study from Madrid, Spain, published in Scientific Reports, suggests that the timing of exposure to nitrogen dioxide (NO2) – a common traffic pollution - may play a major role in the outcome of birth.
The findings show that high NO2 levels during different stages of pregnancy are related to different birth risks. Researchers also found that some women with lower educational status (which is often used to measure socioeconomic status) may be more vulnerable to these adverse effects.
Why Nitrogen Dioxide Matters
Nitrogen dioxide (NO2) is a gas mainly produced by vehicle emissions, industrial activity, and power plants, and is an important indicator of traffic-related air pollution in many urban areas. Long-term exposure to NO2 has been associated with respiratory diseases, cardiovascular disease, metabolic diseases, and lung cancer.
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During pregnancy, scientists believe NO2 might contribute to oxidative stress and inflammation and could damage the placenta and reduce oxygen and nutrients to the developing fetus. While NO2 exposure is linked to low birth weight and preterm birth, the evidence on the most vulnerable stages of pregnancy is not definitive.
Researchers analyzed data from 26,582 live singleton births recorded in Madrid, Spain. Maternal residential addresses were identified and connected with district-level air pollution monitoring stations to estimate average NO2 exposure during each trimester of pregnancy.
The study focused on three major birth outcomes:
Low birth weight (LBW)
Premature birth
High birth weight (HBW)
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Maternal education was also studied for the relationship between pollution exposure and birth outcomes.
Madrid provided a perfect setting for the study as it is one of Spain’s wealthiest regions and it has relatively high NO2 levels, particularly during winter. Some districts also have significant socioeconomic inequalities.
At the first and third trimesters, there are different risks.
One of the most striking findings of the study was that the effects of air pollution varied when exposure occurred during pregnancy.
A higher exposure to NO2 during the third trimester was associated with a higher risk of delivering a baby with low birth weight. In contrast, higher exposure in the first trimester was associated with a lower risk of low birth weight.
It is not a signal that pollution is helpful for early pregnancy; rather, this unexpected finding may be linked to live-birth bias, in which pregnancies that are most affected by pollution may end before birth and not be recorded in birth records. Because the study only considered live births, this is a possibility but has not been confirmed.
For the second trimester, there was no statistically significant association with low birth weight.
Early Pregnancy May Increase Premature Birth Risk
The study found that higher NO2 concentrations in the first trimester showed the strongest evidence of increased risk of premature birth. Higher exposure in the second trimester was also associated with a similar trend, although the statistical evidence was weaker.
Interestingly, the researchers did not find a significant connection between third-trimester NO2 exposure and premature birth.
Premature birth is still one of the leading causes of infant illnesses and long-term developmental complications worldwide, which makes these results very relevant for maternal health policies.
Higher pollution may also result in a higher birth weight.
While much focus is on low birth weight, researchers also found evidence that higher NO2 exposure during the third trimester may increase the likelihood of babies being born with high birth weight.
High birth weight, typically associated with fetal overgrowth or macrosomia, can increase the risk for delivery complications and may be related to maternal conditions such as gestational diabetes. Previous studies have suggested that air pollution may contribute to gestational diabetes, but the Madrid study did not directly assess this condition.
Socioeconomic Factors May Influence Risk
An important aspect of the study was the examination of maternal education as an indicator of socioeconomic status.
In general, women with lower educational attainment appeared to have higher predicted risks of certain adverse birth outcomes, particularly premature birth and low birth weight under higher pollution levels. However, most of the statistical interactions between education and pollution exposure were not consistently significant.
The researchers also found that the apparent protective effect of higher educational attainment decreased as pollution levels increased, suggesting that high pollution may affect pregnant women across different socioeconomic groups.
Sensitivity analyses using maternal immigrant status instead of education showed somewhat different results, indicating that socioeconomic influences are complex and require further investigation.
The Madrid study adds to growing evidence that air pollution during pregnancy might affect fetal development differently depending on the stage of pregnancy. Higher NO2 exposure was associated with higher risks of premature birth, low birth weight, and high birth weight at different trimesters, highlighting the importance of considering pregnancy timing when evaluating environmental risks.
Although more work is needed to confirm the biological mechanisms involved, these findings underscore the need for better air quality policies and more targeted public health interventions to reduce pollution exposure, particularly among vulnerable populations. Improving air quality may not only benefit maternal health but could also contribute to healthier pregnancies and better long-term outcomes for children.
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