# Air Pollution and Preterm Birth: A Scoping Review Focused on Preterm Birth Phenotype and Specific Lengths of Gestation

## TL;DR

This review finds that most studies on air pollution and preterm birth do not break down the different types and lengths of preterm birth, limiting understanding of how pollution affects these outcomes.

## Contribution

The paper highlights a critical gap in the literature by showing that few studies analyze specific preterm birth phenotypes or gestational lengths in relation to air pollution.

## Key findings

- Only 25% of studies report on specific lengths of gestation related to preterm birth.
- Just 6.7% of studies examine specific preterm birth phenotypes (spontaneous or medically indicated).
- Associations between pollutants and preterm birth vary in magnitude and direction across studies.

## Abstract

What are the main findings?
•While there have been over 100 studies of ambient PM2.5 and NO2 with preterm birth since 2011, only one in four studies reports on specific lengths of gestation.•Even fewer (one in fifteen) report on specific preterm birth phenotypes (i.e., spontaneous or medically indicated).

While there have been over 100 studies of ambient PM2.5 and NO2 with preterm birth since 2011, only one in four studies reports on specific lengths of gestation.

Even fewer (one in fifteen) report on specific preterm birth phenotypes (i.e., spontaneous or medically indicated).

What are the implications of the main finding?
•Preterm birth is heterogeneous with respect to lengths of gestation as well as the etiology, but is often analyzed as a single outcome in environmental health studies.•Future studies of air pollution and other environmental exposures with preterm birth should disaggregate preterm birth phenotypes to shed light on potential mechanisms and to focus prevention strategies.

Preterm birth is heterogeneous with respect to lengths of gestation as well as the etiology, but is often analyzed as a single outcome in environmental health studies.

Future studies of air pollution and other environmental exposures with preterm birth should disaggregate preterm birth phenotypes to shed light on potential mechanisms and to focus prevention strategies.

Background/Objectives: Air pollution is a recognized risk factor for preterm birth (PTB), a major cause of neonatal morbidity and mortality. The biological mechanisms underlying this association remain unclear, partly because PTB is a composite outcome that includes both spontaneous (sPTB, from preterm labor or rupture of membranes) and medically indicated (mPTB, for conditions such as preeclampsia or fetal growth restriction) subtypes. Additionally, PTB spans a range of gestational lengths from 20 to 36 completed weeks, which may reflect distinct etiologic pathways. Methods: This scoping review identified studies evaluating two pollutants strongly linked to PTB—particulate matter < 2.5 µm in diameter (PM2.5) and nitrogen dioxide (NO2)—in relation to PTB phenotypes and gestational length. A comprehensive PubMed search using targeted MeSH terms and keywords included studies published between 1 January 2011 and 28 February 2024. Eligible studies examined associations of PM2.5 or NO2 with PTB and were categorized by whether they specified PTB phenotype (sPTB or mPTB), gestational length, or neither. Results: Of 436 eligible studies, 5 evaluated specific PTB phenotypes, 28 considered gestational length, and 3 addressed both. Reported associations of PM2.5 or NO2 with PTB were frequently significant but varied in magnitude and direction. Conclusions: Few studies have examined pollutant exposure in relation to PTB phenotypes or gestational lengths, revealing an important knowledge gap. Standardized approaches to exposure assessment and PTB classification are needed to clarify causal pathways and inform targeted prevention strategies and policies to reduce pollution-related PTB.