Abstract Body: Maternal diabetes is associated with a 4-fold increased risk of offspring developing congenital heart defects (CHDs). It remains unknown how maternal diabetes interferes with cardiac cell lineage determination and leads to malformations in the heart. In this study, we aim to elucidate the cellular mechanisms by which maternal hyperglycemia causes high risk of CHDs in newborns. Here we leverage an in vitro hyperglycemic model using human induced pluripotent stem cells (iPSCs), which could recapture cardiac differentiation and cell lineage commitment during embryonic heart development. We collected differentiating cells at D5 (cardiac mesoderm), D10 (cardiac progenitors), and D14 (early cardiomyocytes) during cardiac differentiation under normal and hyperglycemic conditions, and performed single-cell transcriptomic analysis. We found that hyperglycemia significantly impedes cardiac differentiation of human iPSCs as robust cardiac differentiation is rarely observed in multiple iPSC lines (n=10) under hyperglycemia. At the cellular level, hyperglycemia interferes with cardiac differentiation of human iPSCs in response to WNT signaling activation, which is manifested by reduced number of cardiac mesoderm (MESP1+ PDGFRA+) cells at D5 of differentiation. In contrast, neural differentiation is enhanced under hyperglycemia, with a high proportion of neural cell lineage (SOX2+ PAX6+). At D10, differentiated neural cell lineage dominates the cell population under hyperglycemia at the expense of cardiac progenitors and early cardiomyocytes. At D14, the prevalence of neural lineage persist whereas early cardiomyocytes only accounts for a small portion of cell population under hyperglycemia. Moreover, we treated D30 iPSC-derived cardiomyocytes (iPSC-CMs) with high glucose concentration (25 mM) for 7 days and found that iPSC-CMs show reduced mitochondria respiration and ATP production, but elevated apoptosis and reactive oxygen species (ROS) generation. Together, our data suggest that maternal hyperglycemia could interrupt human embryonic heart morphogenesis through overriding WNT-medicated cardiac differentiation and promoting neural cell lineage determination by default.