CONTEXT: Fetal growth restriction with abnormal umbilical artery Doppler velocimetry (FGRadv), reflective of elevated fetoplacental vascular resistance, is associated with increased risks of fetal morbidity and mortality even in comparison to those of growth-restricted fetuses with normal placental blood flow. One major cause of this abnormally elevated fetoplacental vascular resistance is the aberrantly formed, thin, elongated villous vessels that are seen in FGRadv placentas. OBJECTIVE: The purpose of this study was to determine the role of fetoplacental endothelial cells (ECs) in angiogenesis in normal pregnancies and in those complicated by FGRadv. DESIGN AND PARTICIPANTS: Human placental specimens were obtained from FGRadv and gestational age-matched, appropriately grown control pregnancies for EC isolation/culture and for immunohistochemical studies. Additional mechanistic studies were performed on ECs isolated from subjects with term, uncomplicated pregnancies. MAIN OUTCOME MEASURES: We evaluated tube formation and differential angiogenic gene expression in FGRadv and control ECs, and we used ECs from uncomplicated pregnancies to further elucidate the molecular mechanisms by which angiogenesis is impaired in FGRadv pregnancies. RESULTS: Tube formation assays showed that FGRadv ECs demonstrate fewer branch points and total length compared with those from gestational age-matched controls, and this defect was not rescued by exposure to hypoxia. FGRadv ECs also demonstrated lower aryl hydrocarbon receptor nuclear translocator (ARNT) expression. ARNT knockdown resulted in suppression of key angiogenic genes including vascular endothelial growth factor A expression and led to deficient tube formation. CONCLUSIONS: ARNT expression in the placental vasculature mediates key angiogenic expression and fetoplacental EC angiogenesis, and low ARNT expression in FGRadv ECs appears to be a key factor in deficient angiogenesis. This, in turn, results in malformed thin villous vessels that structurally contribute to the abnormally elevated fetoplacental vascular resistance that is associated with high morbidity and mortality in fetal growth restriction.