 This study is conducted as a prospective, randomized, up-down sequential dose of isobaric ropivacaine in 3 mL that will provide effective analgesia for 50 of parturients in cesarean section. The investigators use the combined spinal-epidural anesthesiaCSEA technique in the study. The initial dose of ropivacaine is chosen as 12 mg in L2-3 group and 15 mg in L3-4 group, and the testing interval is 0.5 mg with subsequent doses being determined by the outcome of the previous injection in the same group. If the previous response is ineffective, the next patient will receive 0.5 mg more than the last patient. If the response of the previous patient is effective, the next patient decrease 0.5mg. 