Comparison of Hydroxyethyl Starch and Crystalloid Coload Combined With Norepinephrine Infusion for Hypotension

Inclusion Criteria: * 18-40 years * Primipara or multipara * Singleton pregnancy ≥ 37 weeks * American Society of Anesthesiologists physical status classification I to II * Scheduled for elective cesarean section under spinal anesthesia Exclusion Criteria: * Body height \< 150 cm * Body weight \> 100 kg or body mass index (BMI) ≥ 40 kg/m2 * Eclampsia or chronic hypertension or baseline blood pressure ≥ 160mmHg * Hemoglobin \< 7g/dl * Coagulation or renal function disorders * Known allergy to hydroxyethyl starch * Fetal distress, or known fetal developmental anomaly