Toxic Trace Element Levels in Maternal and Cord Blood Worldwide and Their Impact on Pregnancy: A Narrative Review.

BACKGROUND:This up-to-date narrative review examines the associations of low environmental exposure to toxic trace elements-arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg)-on pregnancy. While high levels are known to be harmful, the impact of low levels has not been fully analyzed. The aim was to summarize global data on As, Cd, Pb, and Hg levels in maternal and cord blood of healthy pregnant women, compare their distribution, and assess associations with demographic factors, lifestyle, and pregnancy outcomes. METHODS:A systematic literature search was conducted in PubMed, Cochrane Library, and Scopus for studies published between 1 January 1990 and 14 May 2025. The review was conducted in accordance with PRISMA 2020 guidelines. A total of 824 records were assessed for eligibility, and 656 were excluded based on predefined criteria. Exclusion criteria encompassed in vivo or in vitro studies, non-English written publications, treatment-based studies, and studies involving occupationally exposed pregnant women. Inclusion criteria included original full-length research papers with cross-sectional, prospective cohort, or case-control designs; studies measuring As, Cd, Pb, and/or Hg in maternal and/or cord blood; and studies examining associations with demographic, lifestyle, and pregnancy outcomes. The review was not registered, and no external funding was received. Given the narrative synthesis approach of the review, a formal risk-of-bias assessment was not undertaken. RESULTS:A total of 168 studies were included: 32 reported findings on As, 55 on Cd, 78 on Pb, and 64 on Hg, with 26 addressing all four elements. No meta-analysis was performed; results were summarized narratively. Results show that As and Cd levels are higher in maternal blood, whereas Pb and Hg, including methylmercury, are higher in cord blood. Key factors include smoking, rural residence, and fish or seafood consumption, linked to higher Cd, Pb, and Hg levels, respectively. INTERPRETATION:Low Pb levels show the strongest associations with adverse pregnancy outcomes, while As shows the weakest. Pb's effects may relate to passive placental diffusion, unlike other elements. Further studies are needed.