["The Journal of Physiology, EarlyView. ", "\nAbstract figure legend Propionate in the fetal circulation is thought to originate from fermentation of dietary fibre by the maternal gut microbiota. However, the mechanism of transplacental transport has remained unclear. Here, we measured propionate permeability using a placental barrier model generated by differentiating human placental trophoblast stem cells (hTSCs) into syncytiotrophoblasts (SynT) on culture inserts. In this model, transcellular permeability of propionate was observed, and MCT1 was confirmed to function as a propionate transporter on both the apical (maternal) and basal (fetal) sides. By contrast, no evidence supporting the involvement of MCT4 was obtained, based on MCT4 inhibition in the barrier model and assays using MCT4‐expressing systems.\n\n\n\n\n\n\n\n\n\nAbstract\nPropionate, a gut microbiota‐derived short‐chain fatty acid, influences fetal development and postnatal metabolic programming. Although the fetus lacks microbiota and endogenous propionate production, human pregnancies show a fetal‐to‐maternal propionate concentration ratio greater than unity, suggesting concentrative transport across the placenta. However, its underlying mechanism remains undefined. The present study aimed to identify the transporter responsible for transplacental transport of propionate across the syncytiotrophoblast (SynT) layer. Transporter knockdown in human choriocarcinoma JEG‐3 cells revealed that MCT1 (SLC16A1) silencing reduced [3H]propionate uptake, whereas knockdown of other anion transporters had no effect. Functional assays using Xenopus oocytes demonstrated that the expression of human MCT1, but not MCT4 (SLC16A3), increased [3H]propionate transport. In human trophoblast stem cell (hTSC)‐derived SynT, [3H]propionate uptake was pH‐dependent and significantly inhibited by MCT1‐selective inhibitors. Subsequently, to evaluate transcellular transport, we performed quantitative permeability assays using a hTSC‐derived placental barrier model. [3H]Propionate permeability was significantly higher than that of [14C]d‐mannitol, a paracellular marker. MCT1 inhibition reduced [3H]propionate permeability in both apical‐to‐basal and opposite directions, whereas MCT4 inhibition had minimal effects. Notably, the hTSC‐derived model exhibited a directional bias in [3H]propionate transfer, reflecting the fetal‐directed enrichment observed in vivo. Mathematical model analysis further indicated that MCT1 functions at both the apical and basal membranes to facilitate bidirectional transport of propionate. Together, these findings identify MCT1 as the predominant mediator of propionate transfer across the human SynT layer, providing mechanistic insights into how the placenta governs fetal exposure to maternal microbiota‐derived metabolites.\n\n\n\n\n\n\n\n\n\nKey points\n\nThe fetus relies on maternal‐derived propionate for development, but the molecular mechanism responsible for its concentrative transport across the human placenta remains undefined.\nUsing multiple human trophoblast models and functional expression assays, we identified MCT1, but not MCT4, as the primary mediator of propionate transport.\nA human trophoblast stem cell‐derived placental barrier model successfully exhibited a directional bias in propionate transfer, reflecting the fetal‐directed enrichment observed in vivo.\nMathematical modelling of the permeability data from the human trophoblast stem cell‐derived barrier model indicates that MCT1 functions at both the apical and basal membranes of the syncytiotrophoblast to facilitate bidirectional transport.\nThese findings establish MCT1 as a key gateway for transplacental propionate transfer, providing mechanistic insights into how the placenta regulates fetal exposure to maternal microbiota‐derived metabolites.\n\n\n"]