Environmental Exposures and Placental Dysfunction in Pregnancy: A Systematic Review and Mechanistic Synthesis of Microplastics, Per- and Polyfluoroalkyl Substances, Phthalates, Heat Stress, and Air Pollution

Environmental exposures during pregnancy may affect placental function, yet evidence remains dispersed across separate literatures on particles, persistent chemicals, consumer-product toxicants, air pollution, and climate-related heat. This Preferred Reporting Items for Systematic Reviews and Meta-analyses 2020–guided systematic review with mechanistic synthesis examined associations of microplastics/nanoplastics, per- and polyfluoroalkyl substances (PFAS), phthalates, air pollution, and heat stress with placental pathways and adverse pregnancy outcomes. PubMed/MEDLINE, PubMed Central, Scopus, Web of Science, Embase, and the Cochrane Library were searched for English-language studies published from January 2010 to March 31, 2026. Thirty-five studies met eligibility criteria, comprising human observational studies, placental analytical investigations, systematic reviews/meta-analyses, and mechanistic or conceptual reviews. Given substantial heterogeneity in study design, exposure assessment, placental endpoints, and outcome definitions, evidence was synthesized qualitatively rather than pooled statistically, with findings interpreted according to evidence type, directness, and methodological limitations. Associations were most consistent for heat exposure with adverse birth outcomes, PFAS with hypertensive and metabolic pregnancy outcomes, and phthalates with placental endocrine alterations and preterm birth-related outcomes. For microplastics, direct placental detection and translocation evidence was more developed than evidence linking exposure to clinical outcomes. Across domains, oxidative stress, inflammation, endocrine/metabolic disruption, epigenetic programming, and vascular dysfunction emerged as recurrent candidate pathways. These convergent mechanisms support biological plausibility but do not establish causality or multi-exposure synergy. Most studies evaluated single exposure domains, and direct multi-exposure placental evidence remains limited. This review represents a preliminary evidence-mapping step by the contributors to consolidate available data, identify methodological gaps, and guide subsequent longitudinal, placenta-focused, multi-exposure research and prospective data collection.