Objective To investigate the median effective dose (ED50) of dexmedetomidine for maternal loss of consciousness during intraoperative sedation of cesarean delivery under combined spinal and epidural anesthesia． Methods A total of 29 pregnant women, aged 18‒40 years old, at 35‒42 weeks of gestation, with American Society of Anesthesiologists (ASA) grade Ⅰ or Ⅱ, who underwent cesarean section under combined spinal and epidural anesthesia were selected. After delivery of the fetus and placenta, the women were intravenously injected by pump with dexmedetomidine for intraoperative sedation. The dose was determined using the two-stage modified sequential method, with an initial loading dose of 1.4 μg/kg, and the pumping was completed at a constant rate for 15 min. The depth of maternal sedation was assessed by the Observer's Assessment Alert/Sedation (OAA/S) score at the 26th min. The heart rate and mean arterial pressure (MAP) were recorded before administration (T0), 5 min after administration (T1), 10 min after administration (T2), 15 min after administration (T3), 20 min after administration (T4), 25 min after administration (T5), and 30 min after administration (T6). Maternal adverse reactions were also recorded. The ED50 of dexmedetomidine-induced maternal loss of consciousness (OAA/S score≤2) was obtained by calculating the mean of the midpoints of the four turning points in the second stage of the modified sequential method, and the ED95 was calculated by probit regression analysis. Results The ED50 of dexmedetomidine-induced maternal loss of consciousness was 1.94 μg/kg, and the ED95 was 2.11 μg/kg. Compared with those at T0, all women showed decreases in heart rate (all P<0.05) and increases in MAP (all P<0.05) from T1 to T6. There were three cases of bradycardia, three cases of abdominal pain before administration, one case of nausea and chest tightness, which disappeared or were relieved after the administration of dexmedetomidine. No women reported chills or discomfort. Conclusions The dose of dexmedetomidine is high, when the agent is used for intraoperative sedation of maternal loss of consciousness for cesarean section, with obvious hemodynamic fluctuation. Therefore, its clinical application needs to be closely observed.