Micro- and nanoplastics in the fetal ecosystem: A systematic review with Bayesian evidence synthesis on placental translocation and perinatal outcomes

Abstract Background: Micro- and nanoplastics (MPs/NPs) are increasingly detected in human pregnancy-related matrices, including placenta, cord blood, meconium, and amniotic fluid, raising concern about in utero exposure during critical windows of development. Despite rapid growth in this field, evidence on maternal–fetal translocation, underlying biological mechanisms, and perinatal consequences remains heterogeneous and unevenly integrated across study types. Objective: The objective of this study was to systematically synthesize human, animal, and in vitro evidence on MPs/NPs exposure during pregnancy and early life, with a focus on placental translocation, mechanistic plausibility, and reported perinatal outcomes. Methodology: FollowingPreferred Reporting Items for Systematic Reviews and Meta-Analyses 2020 guidelines, we conducted a systematic search of PubMed/MEDLINE, Embase, Scopus, Web of Science, and preprint servers through September 8, 2025, without language restrictions. Eligible studies included human biomonitoring and epidemiologic investigations, controlled animal experiments, and placental-relevant in vitro models. Two reviewers independently performed study selection, data extraction, and risk-of-bias assessment using risk of bias in non-randomized studies of interventions for human studies and Systematic Review Centre for Laboratory Animal Experimentation’s tool for animal studies. Evidence was integrated using a Bayesian qualitative synthesis framework, which weighted findings according to internal validity, cross-model consistency, and translational relevance rather than formal meta-analysis. Results: Among 4,493 screened records, 34 studies met the inclusion criteria (22 primary studies and 12 contextual secondary sources). Human biomonitoring consistently demonstrated the presence of MPs/NPs in placental and fetal compartments across multiple regions. Epidemiologic evidence suggested associations with reduced fetal growth and shorter gestational duration, although inference was limited by small sample sizes and observational designs. Experimental models provided convergent mechanistic support, implicating oxidative stress, ferroptosis, endocrine perturbation, metabolic reprogramming, and neurodevelopmental disruption. Risk-of-bias ratings were frequently moderate, reflecting exploratory scope rather than compromised analytical rigor. Conclusions: This review offers an integrated appraisal of MPs/NPs within the fetal ecosystem, supporting placental translocation and biologically plausible pathways of harm while acknowledging current epidemiologic limitations. The findings underscore priorities for standardized detection methods, prospective pregnancy cohorts, and proportionate preventive strategies spanning clinical practice and environmental policy.