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Background With the global obesity epidemic, rates of maternal diabetes are increasing and offspring exposed to a diabetic environment in utero have an increased risk of developing long-term cardiovascular complications. It is thought that this may be linked to alterations in the fetoplacental macro- and micro-vasculature, although the underlying mechanisms are unclear. Understanding these mechanisms could lead to novel ways to reduce rates of adulthood cardiovascular complications in offspring exposed to a diabetic environment in utero. In this study we assessed whether alterations in the fetoplacental vasculature could be attributed to endothelial-to-mesenchymal transition (EndMT), the transdifferentiation process of endothelial cells into mesenchymal cells.Methods Human placentae were collected at term from patients with maternal (gestational) diabetes (DM; n=18) and those without diabetes (control; n=17). Human umbilical vein macrovascular endothelial cells (HUVECs; n=5–6) were isolated from the umbilical cords and the endothelial phenotype was assessed using flow cytometry. HUVECs were exposed to known EndMT inducers, TGF-β2 (10 ng/mL) and IL-1β (10 ng/mL), for 6 days. Cultured HUVECs and freshly isolated placental villous tissue were fixed or processed for RNA extraction. Immunocytochemistry, immunohistochemistry and RT-qPCR were performed to assess levels of endothelial markers (CD31, Vwf, VE-Cadherin), mesenchymal markers (αSMA, Transgelin, CD73) and EndMT mediators, including two key transcriptional regulators, Slug and Snail.Results Control and DM HUVECs co-expressed CD31 and VE-Cadherin (98.57±0.40% and 99.59±0.24%, respectively). TGF-β2 and IL-1β induced morphological and molecular changes consistent with EndMT in both control and DM HUVECs. The ability of TGF-β2 and IL-1β to alter expression of known EndMT markers, VWF, TGFBR1, IL1B, and IL1R1 was diminished in DM HUVECs, however, all other hallmarks of EndMT were similar. In control and DM placental tissue, Slug and Snail were detected in the villous stroma. Endothelial genes, PECAM1, VWF, and CDH5 were reduced in DM placentas. This was accompanied by a reduction in EndMT regulators, SNAI2, TGFB2, TGFB3 and TGFBR2, however, there was no change in mesenchymal markers or other EndMT regulators.Conclusions Overall, there are limited changes in EndMT in the fetoplacental macrovasculature. Several EndMT mediators were altered in placental villous tissue in DM, suggesting changes in EndMT in the microvasculature, however further work is needed to explore this and to investigate whether this is linked to cardiovascular complications in adulthood.