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

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 (PM_2.5) 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 PM_2.5 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 PM_2.5 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.