Trans fatty acids and pregnancy complications: A systematic review and meta-analysis

Trans fatty acids (TFAs), present in certain processed foods and ruminant products, remain a source of dietary exposure despite global reductions in industrial TFAs (iTFAs). Their relevance during pregnancy is of particular interest because the quality and composition of maternal dietary fats can influence placental function, vascular adaptation, and fetal growth. This review was based on the hypothesis that maternal exposure to specific industrial trans-18:1 isomers is associated with increased risk of pregnancy complications. We systematically evaluated evidence linking individual TFA isomers measured through circulating biomarkers or dietary assessments with major pregnancy complications, including preeclampsia, preterm birth, fetal growth restriction, low birth weight, gestational diabetes mellitus, and stillbirth. PubMed, Scopus, and Cochrane were searched through 2025 for human observational studies and randomized trials assessing maternal TFA exposure using validated methods. Risk of bias was assessed using design-appropriate tools, and a meta-analysis was conducted when possible. Thirteen studies met the inclusion criteria: 11 observational and 2 randomized controlled trials. Evidence was strongest for industrial trans-18:1 isomers. Biomarker-based studies consistently showed that higher proportions of these isomers were associated with increased preeclampsia risk and reduced fetal growth indicators. In contrast, dietary intake studies showed no consistent associations, likely due to their limited capacity to distinguish iTFAs from ruminant TFAs. Evidence for preterm birth and GDM was sparse and inconsistent. Overall, specific industrial trans-18:1 isomers exposure appear most relevant to adverse pregnancy outcomes. Future research should prioritize prospective biomarker-based cohorts and controlled dietary interventions to clarify causality and inform pregnancy-specific nutrition guidance.