Pregnancy Under the Haze: Understanding the Maternal and Fetal Risks of Air Pollution

An Evidence-Based Review of Maternal Exposure to Air Pollution, Placental Dysfunction and Adverse Pregnancy Outcomes, with Particular Reference to Pakistan

Abstract

Background: Ambient air pollution has emerged as an important environmental determinant of maternal, fetal and neonatal health. In Pakistan, particularly in Lahore and other urban centres of Punjab, recurrent winter smog exposes pregnant women to elevated concentrations of fine particulate matter and gaseous pollutants. Increasing epidemiological and experimental evidence suggests that prenatal exposure to air pollution may contribute to adverse pregnancy outcomes through inflammatory, oxidative, vascular and metabolic pathways.

Objective: This narrative review examines the composition of smog, its potential biological effects during pregnancy, associations with obstetric and neonatal complications, and the implications of recent Pakistani research.

Evidence synthesis: International systematic reviews have identified associations between maternal exposure to particulate matter and outcomes including preterm birth, low birth weight, fetal growth restriction and stillbirth. A Pakistani study published in BMJ Open in 2025, involving 9,172 women, reported an association between higher pregnancy-related PM2.5 exposure and increased odds of stillbirth. Proposed biological pathways include oxidative stress, systemic inflammation, endothelial dysfunction and altered placental physiology.

Conclusion: Air pollution should be recognised as an important environmental consideration in maternal healthcare. Although observational associations do not establish individual causation, the accumulating evidence supports preventive exposure-reduction strategies, strengthened environmental monitoring and further investigation into pregnancy-specific biological mechanisms.

Keywords: Air pollution in pregnancy, Lahore smog, PM2.5, maternal health Pakistan, placental oxidative stress, fetal growth restriction, preterm birth, stillbirth, environmental reproductive health.

1. Introduction: Pregnancy in an Increasingly Polluted Environment

Pregnancy represents a period of profound physiological adaptation, during which maternal cardiovascular, respiratory, endocrine and immune systems undergo substantial changes to support fetal development. These adaptations are essential for maintaining placental perfusion, oxygen delivery and metabolic exchange between the mother and developing fetus. However, they may also modify maternal susceptibility to environmental pollutants.

Air pollution has traditionally been discussed in relation to respiratory diseases, cardiovascular disorders and premature mortality. Over recent decades, its potential implications for reproductive health have received increasing scientific attention. The developing fetus is particularly sensitive to disturbances in oxygen availability, inflammatory regulation and placental function, making pregnancy an important period for studying environmental exposure.

In Pakistan, air pollution has become a persistent public-health concern. Lahore experiences recurrent episodes of severe winter smog, frequently associated with a combination of vehicular emissions, industrial activities, combustion sources, atmospheric stagnation and regional pollution transport. Seasonal meteorological conditions, including temperature inversions and reduced atmospheric mixing, can contribute to the accumulation of pollutants near ground level.

A 2026 study examining winter smog in Lahore used satellite observations and socioeconomic survey data to investigate its spatial distribution and public-health implications. The findings reinforce the importance of understanding air pollution as a sustained environmental exposure rather than merely a temporary seasonal inconvenience [1].

For pregnant women, this issue extends beyond respiratory discomfort. Scientific investigations increasingly suggest that inhaled pollutants may initiate systemic biological responses capable of influencing maternal vascular health, placental physiology and fetal development.

The central clinical question is therefore not simply whether smog makes breathing uncomfortable, but whether prolonged or repeated exposure during pregnancy may influence the biological environment in which fetal development occurs.

2. Understanding Smog: Composition and Environmental Sources

Smog is a complex atmospheric mixture rather than a single chemical substance. Its composition varies according to local emission sources, atmospheric conditions, sunlight and seasonal changes.

The principal pollutants of concern include fine particulate matter (PM2.5), coarse particulate matter (PM10), nitrogen dioxide (NO2), sulfur dioxide (SO2), carbon monoxide (CO), ground-level ozone (O3) and various organic compounds.

Among these pollutants, PM2.5 has attracted particular attention in pregnancy research because of its small aerodynamic diameter and capacity to penetrate deeply into the respiratory system.

PM2.5 refers to particles with an aerodynamic diameter of 2.5 micrometres or smaller. These particles may contain carbonaceous compounds, inorganic salts, metals and other substances originating from traffic, fuel combustion, industrial processes and secondary atmospheric reactions.

After inhalation, fine particles can reach the terminal bronchioles and alveolar regions of the lungs. Their interaction with respiratory epithelial cells and immune cells may initiate local oxidative and inflammatory responses. In some circumstances, particle components or ultrafine particles may cross biological barriers, although the extent of direct fetal exposure and its clinical significance remain under investigation.

The World Health Organization's 2021 Global Air Quality Guidelines recommend an annual mean PM2.5 concentration of no more than 5 micrograms per cubic metre and a 24-hour mean of no more than 15 micrograms per cubic metre, using the specified statistical averaging convention [2].

These recommendations are population-level health guidelines rather than absolute thresholds below which exposure is guaranteed to be harmless.

3. Why Pregnancy May Increase Vulnerability to Air Pollution

Pregnancy involves significant changes in maternal respiratory and cardiovascular physiology. Minute ventilation increases, primarily because of an increase in tidal volume, while maternal oxygen consumption also rises to meet the metabolic demands of pregnancy.

Cardiac output increases, plasma volume expands and the uteroplacental circulation develops to support the growing fetus.

These changes do not necessarily mean that every pregnant woman inhales more pollutants than a non-pregnant woman under identical conditions. Nevertheless, they create a distinct physiological setting in which pollutant-related inflammation, oxidative stress or vascular dysfunction may have consequences for both maternal and fetal health.

The placenta plays a central role in this relationship.

As a temporary but highly specialised organ, the placenta facilitates nutrient transfer, respiratory gas exchange, waste removal and endocrine signalling. Its function depends on adequate maternal blood supply, healthy trophoblast development and appropriately regulated immune responses.

Disturbances in these processes may affect fetal growth and pregnancy progression.

Air pollution is therefore being investigated not only as an external respiratory exposure but also as a potential contributor to systemic and placental dysfunction.

4. Biological Mechanisms: How Air Pollution May Influence the Placenta

4.1 Oxidative stress and reactive oxygen species

Oxidative stress occurs when the generation of reactive oxygen species exceeds the capacity of antioxidant defence systems to maintain cellular redox balance.

Airborne particulate matter may promote oxidative stress through several mechanisms, including direct interactions with cellular components, activation of inflammatory cells and disruption of mitochondrial function.

During pregnancy, excessive oxidative stress may affect trophoblast activity, cellular signalling, lipid membranes and vascular regulation.

The placenta normally maintains a carefully regulated oxidative environment. Excessive oxidative stress has been implicated in several placental disorders, although the extent to which air pollution directly causes these disorders in humans remains uncertain.

A systematic review published in Science of the Total Environment in 2025 identified oxidative stress, inflammatory responses and endothelial dysfunction among the biological pathways associated with air-pollution-related adverse pregnancy outcomes [3].

4.2 Maternal systemic inflammation

Inhaled pollutants may stimulate respiratory immune responses, including the release of inflammatory mediators and activation of innate immune pathways.

Inflammatory signalling may extend beyond the respiratory system, potentially influencing maternal vascular function and metabolic regulation.

Investigated mediators include interleukin-6, tumour necrosis factor-alpha and other inflammatory markers. However, biomarker responses vary between studies, and these pathways should not be interpreted as fully established causal mechanisms for individual obstetric complications.

Persistent inflammatory activity may contribute to an altered maternal physiological environment, particularly in pregnancies already complicated by obesity, diabetes, hypertension or other underlying conditions.

4.3 Endothelial dysfunction and uteroplacental circulation

The vascular endothelium regulates blood vessel tone, inflammatory responses and haemostatic balance.

Air pollution has been associated with endothelial dysfunction in cardiovascular research, potentially involving reduced nitric oxide bioavailability, oxidative stress and altered vascular reactivity.

During pregnancy, adequate uteroplacental blood flow is essential for fetal oxygenation and nutrient delivery.

Researchers are therefore investigating whether pollutant-related vascular disturbances contribute to placental insufficiency or hypertensive pregnancy disorders.

Importantly, these mechanisms remain areas of active research and cannot be used to diagnose placental dysfunction solely on the basis of pollution exposure.

4.4 Placental inflammation and cellular injury

The placenta may respond to environmental exposures through changes in inflammatory signalling, cellular stress responses and gene expression.

Experimental investigations have raised questions about particulate accumulation within placental tissues and its possible relationship with placental biology.

However, detecting pollution-associated particles or biological changes in placental samples does not automatically demonstrate that these exposures caused a specific fetal complication.

A distinction must be maintained between the presence of a potential biological mechanism and confirmation of clinically meaningful causation.

4.5 Epigenetic regulation and developmental programming

Epigenetic mechanisms regulate gene expression without altering the underlying DNA sequence.

These processes include DNA methylation, histone modifications and non-coding RNA regulation.

Environmental exposures during pregnancy have been investigated for their potential effects on placental and fetal epigenetic patterns.

Air-pollution-related oxidative stress and inflammatory signalling may influence these pathways, potentially contributing to changes in developmental regulation.

This area is scientifically promising because fetal development involves tightly coordinated patterns of gene expression. Nevertheless, the clinical significance, persistence and reversibility of pollution-associated epigenetic changes remain incompletely understood.

It would therefore be premature to conclude that exposure to smog inevitably produces permanent genetic or epigenetic damage in the developing child.

5. Air Pollution and Adverse Pregnancy Outcomes

5.1 Preterm birth

Preterm birth is defined as delivery before 37 completed weeks of gestation and remains a major contributor to neonatal morbidity and mortality.

Several epidemiological investigations have reported associations between maternal exposure to ambient air pollution and increased preterm birth risk.

A 2024 systematic review and meta-analysis published in Frontiers in Public Health included 49 studies and identified associations between exposure to several ambient pollutants and adverse birth outcomes [4].

Potential biological explanations include inflammatory activation, oxidative stress, vascular dysfunction and altered maternal-fetal signalling.

However, the magnitude of the association differs according to pollutant type, exposure concentration, gestational timing, geographic location and study design.

The available evidence does not support attributing an individual preterm delivery to smog exposure without considering other established obstetric causes.

5.2 Low birth weight and fetal growth restriction

Low birth weight is conventionally defined as a birth weight below 2,500 grams, regardless of gestational age.

Fetal growth restriction, in contrast, refers to a fetus that has not achieved its biological growth potential. The two conditions are related but not interchangeable.

Maternal exposure to air pollution has been associated with low birth weight and small-for-gestational-age births in population-based research.

Potential mechanisms include impaired placental vascular function, altered nutrient transfer, oxidative stress and inflammatory responses.

The World Health Organization recognises associations between maternal air pollution exposure and adverse birth outcomes, including low birth weight, preterm birth and small-for-gestational-age delivery [5].

These findings suggest that environmental exposure should be considered within the broader framework of fetal growth and maternal health, alongside nutrition, maternal disease, socioeconomic conditions and antenatal care.

5.3 Stillbirth: Evidence from Pakistan

Stillbirth is among the most serious adverse pregnancy outcomes and has substantial medical, psychological and social consequences.

A particularly important Pakistani investigation was published in BMJ Open in December 2025 under the title Association of Fine Particulate Matter Exposure During Pregnancy and Stillbirth Rates in Pakistan: A Cross-Sectional Study [6].

The investigators analysed data from 9,172 married women aged 15–49 years who had experienced at least one birth during the preceding five years. Information from the Pakistan Demographic and Health Survey 2017–2018 was linked with satellite-derived estimates of PM2.5 exposure.

The study reported an average estimated PM2.5 exposure of approximately 53.96 µg/m³ across the pregnancy periods assessed.

After adjustment for selected maternal and socioeconomic factors, each 1 µg/m³ increase in estimated average PM2.5 exposure was associated with higher odds of stillbirth, with an adjusted odds ratio of 1.013 (95% confidence interval 1.002–1.025).

This is an important finding because it provides population-level evidence specifically relevant to Pakistan.

However, several limitations require careful interpretation. Pollution exposure was estimated geographically rather than measured continuously for each individual woman. The analysis was observational, and some potential confounding factors, including maternal hypertension and diabetes, were not fully accounted for.

Consequently, the study supports an association between pollution exposure and stillbirth but does not prove that PM2.5 independently caused the observed pregnancy losses.

Nevertheless, the findings strengthen the argument for incorporating environmental health into maternal-health research and public-health planning in Pakistan.

5.4 Hypertensive disorders of pregnancy

Hypertensive disorders, including gestational hypertension and preeclampsia, are major causes of maternal and perinatal morbidity.

Preeclampsia is a complex multisystem disorder involving maternal vascular dysfunction and, in many cases, abnormal placental development or function.

Because air pollution can influence systemic inflammation and endothelial physiology, researchers have investigated its possible relationship with hypertensive pregnancy disorders.

The 2025 systematic review of maternal biological mechanisms identified vascular dysfunction among the pathways potentially linking air pollution with maternal hypertension [3].

However, evidence varies across populations and exposure windows.

Air pollution should therefore be considered a possible environmental contributor rather than an established independent cause of preeclampsia.

Pregnant women should continue routine blood-pressure monitoring and appropriate antenatal assessment regardless of local air-quality conditions.

5.5 Gestational diabetes mellitus

Gestational diabetes mellitus develops when maternal glucose regulation becomes insufficient to meet the metabolic demands of pregnancy.

Inflammatory responses, oxidative stress and altered insulin signalling have been proposed as possible pathways connecting air pollution exposure with metabolic dysfunction.

Some observational studies have reported associations between air pollution and gestational diabetes, although findings are not uniform.

Potential confounding factors include maternal age, body mass index, family history, diet, physical activity and socioeconomic conditions.

At present, exposure to smog is not a substitute for established clinical risk assessment or recommended gestational diabetes screening.

5.6 Fetal neurological and respiratory development

Fetal brain and lung development involves prolonged periods of cellular differentiation, tissue organisation and functional maturation.

Researchers have investigated whether prenatal exposure to air pollution may influence subsequent neurodevelopmental and respiratory outcomes.

Proposed pathways include maternal inflammation, oxidative stress, altered placental signalling and developmental programming.

The WHO notes growing evidence suggesting that air pollution may influence neurological development in children [5].

However, neurodevelopmental outcomes are affected by numerous biological, genetic, nutritional and social factors. Associations observed in population studies should not be interpreted as predictions of neurological impairment in an individual child.

Longitudinal studies with accurate exposure measurement and extended developmental follow-up are required to clarify these relationships.

6. Does the Timing of Exposure During Pregnancy Matter?

Pregnancy is not a biologically uniform period. Different organs and physiological systems develop at different gestational stages.

During early pregnancy, implantation, trophoblast invasion, placental development and embryonic organogenesis are particularly important.

During the second trimester, fetal growth and organ maturation continue, while maternal cardiovascular adaptation and placental circulation remain central to pregnancy progression.

During the third trimester, rapid fetal weight gain, neurological maturation and pulmonary development occur.

Researchers have therefore examined whether pollution exposure during particular trimesters is associated with specific pregnancy outcomes.

Some studies suggest that certain gestational windows may be more susceptible to particular exposures. However, results are inconsistent, and no single trimester can currently be identified as universally safe from the effects of air pollution.

For women living in highly polluted environments, exposure reduction should ideally begin before conception and continue throughout pregnancy rather than being limited to a particular trimester.

7. The Pakistani Context: Why Lahore's Smog Deserves Special Attention

Lahore provides an important setting for investigating the relationship between environmental pollution and maternal health.

Winter smog is influenced by multiple interacting factors, including emissions from transport, industrial activities, combustion sources and atmospheric conditions that restrict pollutant dispersion.

The composition and intensity of pollution may vary between neighbourhoods and across different times of the day.

A 2026 environmental study of Lahore examined winter smog through satellite-based measurements and community-level observations, highlighting its environmental and socioeconomic significance [1].

For pregnant women, exposure may occur during daily commuting, employment, household activities and time spent in poorly ventilated indoor environments.

Importantly, remaining indoors does not necessarily eliminate exposure. Outdoor fine particles may infiltrate homes, offices and healthcare facilities, while indoor combustion and other pollution sources can further affect air quality.

This creates an important equity issue.

Women with limited access to clean indoor environments, appropriate ventilation, air filtration or flexible working arrangements may face greater difficulty reducing exposure.

Maternal health interventions must therefore address both individual protective behaviour and broader environmental conditions.

8. Protecting Maternal and Fetal Health During Smog Season

Although eliminating personal exposure to ambient air pollution is often impossible, several practical measures may help reduce unnecessary exposure.

Monitoring local air quality

Pregnant women and their families should consult reliable air-quality monitoring information, particularly during periods of severe pollution.

The Air Quality Index provides a simplified communication measure based on pollutant concentrations. It is useful for planning activities, although it should not be confused with the actual concentration of PM2.5 measured in µg/m³.

Different AQI systems may use different categories and calculation methods.

Reducing outdoor exposure

During periods of particularly poor air quality, prolonged outdoor exercise, unnecessary travel and activities near heavy traffic may increase inhaled pollutant exposure.

Where feasible, outdoor activities can be rescheduled for periods with better measured air quality.

However, routine antenatal appointments, urgent medical assessments and necessary healthcare should not be postponed solely because of smog.

Respiratory protection

A properly fitted, certified particulate respirator, such as an N95, may reduce inhalation of airborne particles when outdoor exposure cannot be avoided.

The effectiveness of a respirator depends on fit, correct use and the type of pollutant.

N95 respirators do not reliably protect against all gaseous components of smog, and ordinary loose-fitting surgical or cloth masks should not be assumed to provide equivalent particulate filtration.

Pregnant women with significant respiratory or cardiovascular disease should seek individual medical advice regarding respirator tolerance.

Improving indoor air quality

Reducing indoor particle exposure may involve keeping windows closed during severe outdoor pollution episodes, limiting indoor smoke sources and using appropriately sized air purifiers with effective particulate filtration where available.

High-efficiency particulate air (HEPA) filtration can reduce airborne particle concentrations under suitable conditions.

However, filtration effectiveness depends on room size, airflow, filter performance, building leakage and maintenance.

Indoor air management must also account for ventilation needs and any indoor sources of carbon monoxide or other hazardous gases.

Avoiding additional combustion exposure

Tobacco smoke, biomass burning, poorly vented cooking appliances and other indoor combustion sources can contribute to pollutant exposure.

Reducing these sources is especially important in households where ambient pollution is already elevated.

Maintaining antenatal care

Regular antenatal assessment remains essential throughout smog season.

Women with pre-existing hypertension, asthma, cardiovascular disease, diabetes or previous adverse pregnancy outcomes may require individualised monitoring based on their clinical circumstances.

Symptoms such as severe breathlessness, chest pain, persistent severe headache, visual disturbances, vaginal bleeding or reduced fetal movements require prompt medical assessment rather than being attributed automatically to air pollution.

9. Maternal Nutrition and Oxidative Stress: What Can We Conclude?

Because oxidative stress is one proposed pathway linking air pollution with adverse pregnancy outcomes, researchers have considered whether maternal nutritional status influences vulnerability to environmental exposures.

Adequate intake of essential nutrients supports normal pregnancy physiology, antioxidant enzyme systems, cellular metabolism and fetal development.

Nutrients such as vitamin C, vitamin E, selenium, zinc and other dietary components participate in physiological antioxidant processes.

However, a critical scientific distinction must be maintained.

The involvement of oxidative stress in pollution-related biological pathways does not establish that antioxidant supplements prevent air-pollution-associated pregnancy complications.

At present, there is insufficient high-quality clinical evidence to recommend antioxidant supplementation specifically as a treatment or preventive intervention against smog-related fetal harm.

Excessive supplementation may itself carry risks, particularly during pregnancy.

Pregnant women should therefore follow established nutritional recommendations, maintain a balanced diet and use prenatal supplements according to individual clinical needs and professional guidance.

Nutritional care is an important component of maternal health, but it cannot replace environmental pollution control.

10. Research Gaps and Future Directions

Despite increasing international evidence, important research gaps remain in Pakistan.

Many studies rely on satellite-based exposure estimates, fixed monitoring stations or residential location rather than direct measurement of personal pollutant exposure.

Such methods are valuable for large population studies but may not accurately capture time spent indoors, commuting patterns, occupational exposures or changes in residence.

Future research could combine personal exposure monitoring with longitudinal maternal and fetal assessments.

Prospective pregnancy cohorts in cities such as Lahore could investigate associations between trimester-specific PM2.5 exposure and serial fetal growth measurements, maternal blood pressure, inflammatory biomarkers and birth outcomes.

Placental histopathology and molecular investigations may help clarify potential mechanisms of vascular injury, oxidative stress and inflammatory responses.

Another promising area involves studying the combined effects of air pollution, environmental metals, maternal nutritional status and socioeconomic vulnerability.

Epigenetic investigations using placental tissue or umbilical cord blood may provide insights into developmental responses to environmental exposure, although such studies require rigorous laboratory methods and careful interpretation.

Equally important is the need for research evaluating practical interventions.

Studies assessing indoor filtration, exposure-reduction counselling, occupational modifications and public-health measures could help determine which approaches produce meaningful improvements in maternal exposure and clinical outcomes.

Ultimately, multidisciplinary collaboration between obstetricians, environmental scientists, epidemiologists, toxicologists and public-health specialists will be essential.

11. Conclusion

Air pollution is increasingly recognised as an important environmental consideration in pregnancy.

The growing body of scientific evidence suggests that maternal exposure to fine particulate matter and other ambient pollutants is associated with adverse outcomes, including preterm birth, low birth weight, impaired fetal growth and stillbirth.

Potential biological mechanisms involve oxidative stress, inflammation, endothelial dysfunction and altered placental physiology. However, important uncertainties remain regarding individual susceptibility, causal pathways and the relative contribution of specific pollutants.

Recent Pakistani research provides particularly relevant evidence, including a nationwide analysis linking estimated pregnancy-related PM2.5 exposure with higher odds of stillbirth.

For cities such as Lahore, where seasonal smog represents a recurring environmental challenge, these findings have important implications for maternal healthcare and public-health policy.

Protecting pregnancy requires more than clinical surveillance alone. It also requires recognition of environmental exposures that may influence maternal and fetal health.

Reducing air pollution, improving exposure awareness and strengthening evidence-based maternal care are complementary priorities.

A healthy pregnancy depends not only on the care a mother receives, but also on the environment in which she lives and breathes.

Scientific References

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2. World Health Organization. WHO Global Air Quality Guidelines: Particulate Matter (PM2.5 and PM10), Ozone, Nitrogen Dioxide, Sulfur Dioxide and Carbon Monoxide. Geneva: WHO; 2021. https://www.who.int/publications/i/item/9789240034228

3. Maternal mechanisms in air pollution exposure-related adverse pregnancy outcomes: A systematic review. Science of the Total Environment. 2025;970:178999. https://doi.org/10.1016/j.scitotenv.2025.178999

4. Exposure to ambient air pollutions and its association with adverse birth outcomes: A systematic review and meta-analysis of epidemiological studies. Frontiers in Public Health. 2024. https://doi.org/10.3389/fpubh.2024.1488028

5. World Health Organization. Air Quality, Energy and Health: Health Impacts. https://www.who.int/teams/environment-climate-change-and-health/air-quality-energy-and-health/health-impacts

6. Hameed W, Usmani BA, Rehman SU, et al. Association of fine particulate matter exposure during pregnancy and stillbirth rates in Pakistan: A cross-sectional study. BMJ Open. 2025;15. https://doi.org/10.1136/bmjopen-2025-100972

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Medical Disclaimer

This article is intended solely for educational and scientific information. It is based on published research and public-health guidance and should not be interpreted as personalised medical advice, a clinical diagnosis or a substitute for consultation with a qualified obstetrician or healthcare professional.

Associations reported in observational research do not necessarily establish a direct cause-and-effect relationship. Individual pregnancy risks depend on multiple medical, environmental, genetic and socioeconomic factors.

Pregnant women should not initiate medications, supplements or other interventions specifically to prevent pollution-related complications without appropriate medical advice. Anyone experiencing concerning maternal symptoms or changes in fetal movements should seek timely professional assessment.

Editorial and scientific verification

The article is structured as a narrative scientific review, not an original experimental study. Its central findings are supported by the Pakistani BMJ Open study, the WHO air-quality guidance and peer-reviewed systematic reviews.