Hypotension resulted from neuraxial block is a common problem, of which is a special issue in patients undergoing Cesarean section. A large number of studies and clinical guidelines suggest that fluid loading, pre- or co-anesthesia, is a promising manner in preventing hypotension. However, it is still a controversy because the fact of a relatively increased blood volume in parturients. In addition, although it is effective of fluid management, it's precise relationship between fluid (crystalloid or colloid) volume and the proportion of hypotension in Cesarean patients under neuraxial anesthesia is still unknown. The investigators designed this trial to clarify the accurate relationship between fluid volume in an escalated manner and the occurrence of hypotension analyzed with a non-linear regression, and wanted to present the 50% effective volume (EV50) of fluid including crystalloid and colloid in preventing hypotension in patients undergoing Cesarean section.

Inclusion Criteria:

• 21-40 yr
• First time of delivery
• ASA status I-II
• No premature
• No genetic and infectious diseases
• Chinese

Exclusion Criteria:

• < 21 yr
• > 40 yr
• Subjects with cardiac and pulmonary disorders
• Dislocation of placenta
• Pregnant hypertension
• Allergy to local anesthetics
• Unwilling to cooperation
• Need intraoperative administration of vascular active agents
• With significant delivery side effects
• With contradictions of neuraxial anesthesia