Norepinephrine and Phenylephrine for Spinal Anesthesia-induced Hypotension.

Inclusion Criteria: * 18-45 years * Primipara or multipara * Singleton pregnancy ≥37 weeks * American Society of Anesthesiologists physical status classification II * Scheduled for 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 ≥140 mmHg * Hemoglobin \< 7g/dl * Fetal distress, or known fetal developmental anomaly