Inclusion Criteria:  Singleton pregnancy at term scheduled to be delivered via elective cesarean delivery; height 150-180 cm; American Society of Anesthesiologists (ASA) physical status II-III; body mass index (BMI) <35 kg/m2.  Exclusion Criteria:  transverse presentation, fetal macrosomia; uterine abnormalities (eg, large fibroids, bicornuate uterus); polyhydramnios; ruptured membranes, oligohydramnios; intrauterine growth restriction; gestational or nongestational hypertension, diabetes, or eclampsia; hypertensive disorders or any condition associated with autonomic neuropathy (such as diabetes mellitus for >10 years) or renal failure; contraindications for combined spinal-epidural anesthesia; participants who declined to sign informed consent forms.